WORKING TOGETHER

TO HELP PROTECT THE

WORLD’S FRESHWATER
RESOURCES

Conservation Climate Change Sustainability

WWF WATER SECURITY SERIES 2
KEEPING RIVERS ALIVE
A PRIMER ON ENVIRONMENTAL FLOWS AND THEIR ASSESSMENT
 Acknowledgements
WWF’s Water Security Series sets out key concepts This report was prepared by Pegasys Consulting.
in water management in the context of the need for The authors would like to thank Andy Wales, Jason
environmental sustainability. The series builds on lessons Morrison, Will Day, Lauren Iannarone, Richard Mattison
from WWF’s work around the globe, and on state-of- and Denise Knight for their insight and comments to
the-art thinking from external experts. Each primer in early drafts.
the Water Security Series will address specific aspects
The authors would like to acknowledge the series
of water management, with an initial focus on the
editors:Dave Tickner, Tom Le Quesne and Mica Ruiz.
inter-related issues of water scarcity, climate change,
infrastructure and risk.

Understanding Water Security

As an international network, WWF addresses global
threats to people and nature such as climate change,
the peril to endangered species and habitats, and
the unsustainable consumption of the world’s natural
resources. We do this by influencing how governments,
businesses and people think, learn and act in relation
to the world around us, and by working with local
communities to improve their livelihoods and the
environment upon which we all depend.

Alongside climate change, the existing and projected

scarcity of clean water is likely to be one of the key
challenges facing the world in the 21st Century. This
is not just WWF’s view: many world leaders, including
successive UN Secretaries General, have said as much
in recent years. Influential voices in the global economy
are increasingly talking about water-related risk as an
emerging threat to businesses.

If we manage water badly, nature also suffers from

a lack of water security. Indeed, the evidence is that
freshwater biodiversity is already suffering acutely
from over-abstraction of water, from pollution of rivers,
lakes and groundwater and from poorly-planned water
infrastructure. WWF’s Living Planet Report shows that
declines in freshwater biodiversity are probably the
steepest amongst all habitat types.

As the global population grows and demand for food and

energy increases, the pressure on freshwater ecosystems
will intensify. To add to this, the main effects of climate
change are likely to be felt through changes to the
hydrological cycle.

WWF has been working for many years in many parts of

the world to improve water management. Ensuring water
security remains one of our key priorities.

February 2009

Jay O’Keeffe, Tom Le Quesne

CONTENTS

04 SUMMARY 28 PART D
Environmental flows and their assessment Assessing environmental flows

06 INTRODUCTION 29 Methods for the assessment

Why do we need environmental flows? of environmental flows
33 Indicators
07 PART A
Rivers, flows and 36 ANNEX A
environmental requirements Objectives hierarchy – A method for setting
08 Rivers and flows measurable indicators to achieve a vision
11 What are environmental flows?
37 ANNEX B
12 PART B Defintions of some terms used
The basis of environmental flows in this document
13 The ecological assumptions for
environmental flows 38 ANNEX C
15 Limitations of environmental flows Further reading

17 PART C 39 References used in the text

Environmental flows: Key issues
18 Implementation
20 A scientific and social process
21 Choosing a suitable assessment method
23 Setting objectives
26 Policy and legislation on environmental
flow assessment

3
 Summary:
Environmental flows
and their assessment

Increasing global exploitation of water resources has led differing approaches for a river in a protected area in
to significant degradation of freshwater biodiversity and contrast to a river in a major irrigation or urban area.
the services that rivers provide. In many places, rivers Choice and judgement, particularly when deciding on
have completely stopped flowing. environmental objectives, are an essential part of the
environmental flow process.
The socio-economic consequences of disruption to and
collapse in freshwater systems are often profound: people 3. Environmental flow assessment is based on the
are much more dependent on natural riverine services assumption that there is some ‘spare’ water in rivers
than is immediately apparent, and this only becomes that can be used without unacceptably impacting on
obvious when the river is seriously degraded. the ecosystem services that the river provides.

In response to this, the concept of environmental 4. Environmental flows are not just about establishing a
flows has been developed over recent decades. An ‘minimum’ flow level for rivers. All of the elements of
environmental flow is an amount of water that is kept a natural flow regime, including floods and droughts,
flowing down a river in order to maintain the river in a are important in controlling the characteristics and
desired environmental condition. As the concept has natural communities in a river. For example rivers
evolved, there has been significant development of with a constant flow regime can quickly become
approaches to the assessment of environmental flows. dominated by pest species.

From global experience with environmental flow 5. Environmental flows don’t always require an increase
assessment in recent decades, a number of key from present flows. In some cases, e.g. where low
lessons can be drawn. season flows have been artificially increased by inter-
basin transfers or releases from dams for hydropower,
1. The characteristics and ecosystems of rivers are
the environmental flow recommendations may be
controlled in a very significant way by physical
for lower flows than present. In other cases, the
processes, in particular flows. An environmental flow
assessment may identify that some further water can
regime describes all the different flows (wet season,
be abstracted without unacceptable impacts.
dry season, floods, droughts etc) that are needed to
keep the river and all its aspects functioning in 6. Environmental flow assessments are not just useful
a desired condition. on rivers for which the water resources have been/
are being developed – it’s very useful to know the
2. Environmental flow assessment is both a social and
environmental requirements before any development
a scientific process, with a social choice at its core.
plans are made, so that these flows can be factored
There is no one correct environmental flow regime
into the planning process at an early stage.
for rivers – the answer will depend on what people
want from a river. Different sorts of rivers are likely to
have different requirements and priorities, for example
4
Summary
Environmental flows and their assessment

7. There are now over 200 methods for assessing 10. Lack of information, and lack of resources, should
environmental flows. Some are very quick modelling never be a barrier to some implementation of
or extrapolation methods, requiring no or minimal environmental flows. Some attempt to restore some
extra work; others require years of fieldwork and of the natural flow variability is always better than
specialists from a number of disciplines. The none, and fine-tuning is always possible as more
five main categories of assessment are: look- knowledge and resources become available.
up table approaches; extrapolation approaches;
11. In almost any context, implementation presents
hydraulic rating methodologies; habitat simulation
an immeasurably greater challenge than assessing
methodologies; and, holistic methodologies.
the necessary flows. Conservationists therefore
The choice of method will depend on:
need to ensure that they do not devote a
• The urgency of the problem disproportionate amount of effort to discussion and
debate over the appropriate environmental flow
• Resources available for the analysis
methodology, neglecting the more important effort
• The importance of the river of working for implementation.
• Difficulty of implementation

• The complexity of the system.

8. Instead of undertaking extensive prior assessment,

an important alternative approach may often be to
concentrate immediately on implementation of some
flows. This option then requires careful monitoring
of the results of trial flows, to see whether they meet
objectives. This is likely to be particularly important in
situations where there is already an acute problem of
over-abstraction.

9. In all contexts, implementing environmental flows

should be an adaptive process, in which flows may
be successively modified in the light of increased
knowledge, changing priorities, and changes in
infrastructure (e.g. removal of dams) over time.
From this perspective, it may be more appropriate
for legislation to require implementation but allow
flexibility in the methods of assessment.
5
 Introduction:
Why do we need
environmental flows?

It is becoming increasingly evident that, on regional and In order to decide on an environmental flow, people need
global scales, freshwater biodiversity is more severely to decide what they want a river to do for them. Do they
endangered than that of terrestrial or marine systems. want to grow crops, or to generate electricity, or to supply
Freshwater systems are home to 40% of all fish species towns with water, or to keep it in a national park? The
in less than 0.01% of the world’s total surface water, second decision that people have to make is: what state
and when water-associated amphibians, reptiles and do they want the river to be in? In most cases they want
mammals are added to the fish totals, they together to make use of the water and other resources of the river,
account for as much as one third of global vertebrate so they don’t want to keep it entirely natural. Also, in most
biodiversity. Even at a conservative estimate, there have cases (all cases hopefully) they don’t want to turn it into
been global population declines of freshwater vertebrates a dry river bed or a drain for waste. So they have to
averaging 55% between 1970 and 2000. decide in what state between natural and completely
ruined they would like to keep it. This is the role of
At the same time, people need to use rivers, lakes and
environmental flow assessment.
wetlands for many things – water to drink, irrigated food,
industrial water supply, water repurification, fishing,
boating, recreation and cultural activities. If we are careful,
rivers can do all these things for us, but more and more,
people only see rivers as suppliers of water and as
drainage ditches. So lots of rivers round the world now
stop flowing, and others carry only waste water. Like
other natural resources, rivers are very useful if they
are used sensibly, and useless or dangerous if they are
abused. Rivers with little or no flow, and lots of waste
water, are likely to become centres of diseases like
malaria, cholera, schistosomiasis and dysentary.

Environmental flows are aimed at keeping at least some

of the natural flow patterns along the whole length of a
river, so that the people, animals and plants downstream
can continue to survive and use the river’s resources.
So environmental flows are really about using water
resources fairly.

6
Part A:
Rivers, flows
and environmental
requirements

7
 PART A:

Rivers and flows

• R
 iver habitats are generally controlled by physical processes (flow, water quality, sediment
transport) so we can make big changes to the biodiversity of rivers by managing
(or mis-managing) the flow.
• R
 ivers have all sorts of flow patterns – some flow permanently, some seasonally, and some
desert streams only once every few years. The biodiversity of different rivers, and different
parts of rivers, varies with these physical patterns

Flows – the main driver of biodiversity in rivers “During recent decades, scientists have amassed
considerable evidence that a river’s flow regime – its
Rivers, except for a few very large ones, are very variable
variable pattern of high and low flows throughout the
and unpredictable, so the animals and plants that live in
year, as well as variation across many years – exerts
them have to be able to deal with sudden extremes such
great influence on river ecosystems. Each component
as floods and droughts. As a result, most river ecologists
of a flow regime – ranging from low flows to floods – plays
agree that the communities of animals and plants found
an important role in shaping a river ecosystem. Due to
in riverine ecosystems are largely controlled by physical
the strong influence of a flow regime on the other key
rather than biological processes.1
environmental factors (water chemistry, physical habitat,
So, if we want to maintain freshwater biodiversity, then it biological composition, and interactions), river scientists
is necessary to manage the physical processes in rivers refer to the flow regime as a master variable.”
properly. And what are these physical processes? They
include water quality, sediment dynamics, and, of course, Types of flow regime
flow. Flow is the main driver of biodiversity in rivers – it
It is important to recognise that there are as many types
creates the aquatic habitats, it brings the food down from
of flow regimes as there are rivers:
upstream, it covers the floodplain with water during high
flows, and it flushes the sediment and poor water quality • There are rivers that flow all year (perennial rivers),
through the system. Bunn and Arthington (2002) suggest those that flow only during the wet season (seasonal
the principles that: rivers), and those that very rarely flow at all (ephemeral
rivers or desert streams)
1F
 low is the major determinant of physical habitat
in streams; • There are mountain streams – steep, rocky and clear;
there are foothill streams – alternating pools and
2R
 iverine species have evolved primarily in response
rapids; there are floodplain rivers – meandering,
to natural flow regimes;
mud-bottomed, with riparian wetlands
3F
 low connectivity throughout the river and its floodplain
• There are flashy rivers – prone to flood and
are essential to the maintenance of riverine populations;
drought; and there are reliable rivers – spring
and
fed for constant flow.
4A
 ltered flow regimes facilitate the growth and spread of
introduced species.

A recent World Bank (2008) document has characterised

flows as a ‘master variable’ in freshwater systems:

1 Consider, by way of contrast, a tropical forest. Biological communities in such a forest are significantly determined by interaction with each other,
for example the trees that create the habitats for mammal and bird species.

8
PART A:
Rivers and flows

All of these river channels have been formed over The type of channel and floodplain also dictates the
geological time by their flow regimes: some have cut physical habitats that will be available for the flora and
deeply through the mountains and some have carved out fauna of the river: mosses and algae, agile insects such
wide smooth floodplains. It is a long-term function of the as mayflies and stoneflies, and fast-swimming fish like
flows is to make the shape and texture of the channel that trout predominate in the rocky rapids and clear water of
they run in. They do this by eroding the landscape and by upper streams; lilies, reeds, worms and snails, catfish and
carrying the resulting sediment downstream and depositing carp dominate the slower-flowing turbid lowland rivers.
it somewhere else in the channel. So clear fast-flowing The variety and abundance of life in a river (its biodiversity)
water results in rocky or stony channels, while slow-flowing, will be governed by the diversity of habitats in time and
sediment laden water results in muddy channels. space, and therefore largely by the flow regime.

Figure 1 Different flow patterns charcterised in graphs of daily flows over a number of years.
River Flows

River Flows

a) 12 months c) 12 months

J F M A M J J A S O N D J F M A M J J A S O N D

a. Groundwater-fed Creek. c. Snowmelt River

River Flows

River Flows

b) 12 months d) 12 months

J F M A M J J A S O N D J F M A M J J A S O N D

b. Flashy River. d. Winter rain.

9
 PART A:
Rivers and flows

Contrasting Case studies of modified flow The Great Fish River:

regimes from South Africa
The Great Fish River rises in the Karoo region of the
Environmental flow regimes attempt to mimic as much Eastern Cape in South Africa, and flows into the sea
as possible of the natural variability of the flow regime some 200 km east of Port Elizabeth. A naturally seasonal
in rivers. These two case studies show the contrasting river, it stopped flowing during the dry season in most
effects of reducing the flow in a permanently-flowing years. That was until 1975, when a pipeline and canal
river, and increasing the flow in a seasonal river, As these system brought water from the Orange River into its upper
cases demonstrate, environmental flows are not just about reaches, to provide irrigation water and to augment the
maintaining minimum flow levels in rivers. water supply in Port Elizabeth. Since then, the river has
flowed permanently at between 3 and 5 m3s-1, apart from
The Letaba River: occasional higher floods.
The Letaba River rises on the Drakensberg escarpment River ecologists in the early 1970’s who were worried
and flows through the “lowveld” region of north-eastern about the possible effects of the water transfer took
South Africa, into the Kruger National Park, where it joins samples of fish, water quality and invertebrates, so that
the Olifants River flowing into Mozambique. Until the it was possible to repeat these after the transfer had
late 1960’s the river flowed permanently, but gradually it begun. Sampling in the 1980’s showed that a number
stopped flowing during the dry season, due to upstream of non-native fish species had been introduced into the
impoundment and irrigation. In the 1980’s and 90’s the river, probably via the water transfer scheme, and were
river stopped flowing every year, and during the worst significantly affecting the natural fish community. The
droughts of 1982/83 and 1991/92 there was no surface invertebrate species had also changed: one species,
water at all in long stretches of the river. Studies of the which had been very rare in the river prior to the transfer,
biodiversity of the river in the Kruger National Park showed had become so dominant that it constituted more than
that the number of fish in the river was considerably 98% of the total invertebrate community in the middle
lower than in the neighbouring Sabie River, and that reaches of the river. This species, a blackfly called
the invertebrate diversity comprised only 60% of that in Simulium chutteri, has a similar life cycle to mosquitoes,
the Sabie – made up of the most common and hardy with the larvae and pupae growing in the water, and the
species. Habitat for hippos and crocodiles in the Park was adult females requiring a blood meal to facilitate egg
also drastically reduced. Apart from these indicators of development. During Spring, millions of these flies emerge
environmental degradation, the river no longer provided from the river, and settle on the local farm animals, causing
a reliable water supply for the rural communities living next millions of dollars worth of damage and disturbance.
to the river, many of whom relied entirely on the river for
This is a classic example of the effect of removing the
their water. The river also failed for much of the time, to
natural variability, or disturbance regime, of a river. The
provide any flow into the Olifants River, itself highly polluted
water transfer has created homogenous habitat conditions
by mining activities. This further reduced the volume and
throughout the year, and these happen to favour the
quality of water flowing downstream into Mozambique.
blackfly, and the introduced fish species, over the rest
of the natural biodiversity.

10
 PART A:

What are
environmental flows?
• R
 ivers are being used for many purposes, and dams, canals etc are built in rivers to change
the flow regime. These changes affect the natural goods and services that we get from rivers,
often to our detriment
• S
 o, we have to decide how much we want to change them, and how much of the natural
services we would like to keep. The emerging science of environmental flows aims to provide
the balance between the use and the protection of natural water resources for people
• E
 nvironmental flow scientists use ecological components and processes (fish, plants, water
quality etc) as the most sensitive indicators of the state of the river.

There is a growing recognition of the importance of the a particular suite of ecological conditions, for example
natural functions that rivers provide, and the value of the flows to maintain river channels or allow for migration of
biodiversity that lives in or is dependent on them. The particular species.
natural functions and biodiversity of rivers provide goods
Environmental flows scientists use ecological and social
and services that we can take for granted: a reliable water
indicators to measure the state of the river. Very often a
supply, fish and other foods, water purification, transport
fish which is sensitive to flow conditions, insects which
of nutrients downstream and onto fertile floodplains
lay eggs in the river, or water chemistry will give the first
and deltas, and significant cultural and recreational
indication that the river is changing. This doesn’t mean
values. If we wish to maintain these vital functions and
that these indicators are the exclusive, or even the main
the biodiversity associated with freshwater, then it is
reason for managing the river in a particular way. Like coal
necessary to maintain the flow regime that fundamentally
miners used to take canaries down the mine, because
supports them.
they are particularly sensitive to low oxygen levels,
If we accept that water resources have to be developed ecological components of a river can be used to indicate
to provide for human needs, then we also have to accept incipient problems, before they reach the stage
that we are not going to be able to maintain completely of endangering people and their livelihoods.
natural flows, and this will have consequences for the
Rivers can do many things for people: but they can’t do
functions and biodiversity of river systems. The questions
all these things, all the time, for everybody. People have
then are: how much change are we prepared to accept?
to choose. So, for communities that are not content to see
How much can the flow regime be modified before we
their natural resources over-controlled and over-exploited,
create conditions that we don’t want to live with? How
but want to get the best out of the artificial uses and
much water does a river need?
the natural goods and services, environmental flows are
The emerging science of environmental flows has becoming an integral part of a sustainable management
developed to assess the consequences of changing plan for water resources.
flow regimes on aquatic ecosystems. These
assessments require an understanding of the chain of
events which result from altered flows – from climate
change to hydrology, hydraulics, geomorphology and
water chemistry, to ecological and socio-economic
consequences. Environmental flow assessment examines
different scenarios of water resource development, and
predicts the chain of consequences. Environmental flow
assessment can also assist in developing modified flow
regimes that are custom-designed to achieve or maintain
11
 Part B:
The basis of
environmental flows

12
 PART B:

The ecological assumptions

for environmental flows
• T
 he current approach to the assessment and implementation of environmental flows is based
on two key overarching assumptions:
• Some water can be taken out of a river without unacceptably reducing the services which
the river provides
• T
 he flow variability (high flows, low flows, floods etc) provides a range of conditions
which maintain the biodiversity of rivers, and prevent the dominance of individual species
(which are often pests). All the elements of the natural flow regime, including droughts, are
important in controlling the natural communities of a river.

Assumption 1: There is spare water in rivers of a river. This flow variability is an important part of what
ecologists recognise as the “disturbance regime” of
If the water resources of a river are used, then there will
rivers, which is considered to be an important process
by definition be less water left in the river, and this will,
that maintains biodiversity. Different flows are responsible
to a greater or lesser extent, affect the character of the
for creating varying habitat conditions in time and space
river ecosystem. However, rivers are variable systems,
(stream patchiness). These habitat conditions in turn
and the species that live in them are resilient to changes.
provide for a diversity of niches and refugia under all
Annual flows in rivers may vary by orders of magnitude
conditions. The maintenance of such a diversity of
from year to year – very large floods in some years, and
habitats ‘spreads the risk’ for different species at the
extreme droughts in others. By implication, any species
community and population level.
which persists in such a river must be able to survive
during years when there is much less water than average. Such is the importance of the disturbance regime
Providing conditions do not drastically depart from in rivers, that Resh et al. (1988) conclude:
those that have occurred in the past, the animals and “ ... disturbance is not only the most important feature of
plant populations in the river will fluctuate around some streams to be studied, it is the dominant organising factor
common condition. As a consequence a lower than in stream ecology.”
normal flow regime which still incorporates all the major
These are some of the most widespread effects
features of the natural regime may not significantly change
of different types of flows in rivers:
the biota.
• Base flows in the river maintain the in-channel
In addition all rivers do not necessarily need to be
habitats – the rapids, stony riffles, pools, marginal
maintained in a near pristine condition. In fact, most have
areas of tree roots and reeds, backwaters, chutes,
already been modified in many ways. The recognition
runs and waterfalls that are all inhabited by different
that most rivers are no longer pristine, and the setting of
animals and plants. It’s very important that these base
achievable conservation goals tailored to specific rivers,
flows vary from season to season, allowing new and
allows an assessment of the amount of water that can be
different species to flourish and breed.
abstracted without compromising the chosen objective.

Assumption 2: Flow variability and the natural

disturbance regime of a river is important for the
maintenance of its biodiversity
The old notion of a ‘minimum flow’ necessary to keep
the biota alive has fallen away, and in its place is
the recognition that elements of all types of flow are
necessary to provide the dynamic ecological equilibrium
13
 PART B:
The ecological assumptions for environmental flows

• Contrary to popular perception, droughts are equally such as water hyacinth etc. So the full range of flows are
important for biodiversity – limiting the fast-swimming important to maintain the habitats and biodiversity that
predators and allowing more sedentary plant and ultimately provide the goods and services that we value
animal species to have their turn. from the river.

• Higher flows and floods have different functions: Assessments of the flows necessary to keep the river
they scour out the old sediments and bring in fresh in the desired conditions have to take account of all this
nutrient-laden supplies; they maintain the channel diversity. The main lesson for environmental flows is that
shape, and inundate the floodplains, replenishing their effect can only be judged against the natural flow
the wetlands which carry their own unique flora and regime of the river – the worst changes can be to stop a
fauna. Small floods at the beginning of the wet season perennial river flowing or to regulate a seasonal river so
provide cues for fish to begin upstream migrations, that it flows all the time (see the South African examples
and for insects to pupate and emerge as aerial adults. described in the box above).
The very large floods which may occur on average
once every century can act as ‘resetting’ mechanisms
– carving out new channels and sweeping away the
debris of decades. The result may be almost a new
river, and the flows that follow will gradually repeat the
processes of encroachment and accumulation before
the next big flood.

So, rivers are extremely dynamic systems at a whole

range of spatial and temporal scales – upstream and
downstream; within a river reach; across the floodplain or
riparian zone; seasonally, between wet and dry years and,
between the occurrence of major floods.

A regulated river which flows at the same rate all the time
will soon create the same habitats throughout its length,
and a few species suited to the unchanging conditions will
multiply at the expense of others. We usually call these
pest species – like mosquitoes, blackfly, floating plants

14
 PART B:

Limitations of
environmental flows

• F
 lows control many, but not all, processes in rivers. Environmental flows can only achieve
their objectives if land-use, effluent disposal and other non-flow related processes are
also controlled.

Although flows are a dominant governing process in Pollution problems caused by poor land management
rivers, there are many problems which are not related to should be addressed at source by improving land
the flow regime, and which need to be solved by methods management, and not by flow allocation; similarly,
other than flow manipulation. It is important to recognise problems caused by introduced species (fish or plants)
that environmental flows can only help to maintain or cannot be solved by changing flows, although a
improve conditions in a river if the other non-flow related healthy flow regime will usually favour indigenous
aspects are also looked after. species over invasives.

© J REZAC / WWF-UK

The River Kennet, England. The river suffers from both overabstraction for public water supply, and industrial
and agricultural pollution.

15
 PART B:
Limitations of environmental flows

Here are a number of different but typical types of Water quality and environmental flows
environmental problems, with comments about the
There are two views of the role of water quality in environ-
role of environmental flows:
mental flows. One is that water quality issues are an integral
• Loss of riparian vegetation due to over-grazing: This part of the environmental flow assessment process. In this
should be addressed at source, by reducing grazing view, environmental flows must take account of all water
pressure. High level environmental flows to over- quality issues, so that flows may have to be maintained to
top the banks could encourage the regeneration of prevent existing pollution from irrigation return flows, from
riparian plants, but these would not survive if over- industrial effluent or from treated or untreated sewage
grazing continued from having an unacceptable impact. Allowing increased
abstraction will lead to unacceptable impacts because of
•  ater quality problems due to sewage effluent:
W
the consequent increased concentration of pollution.
These should ideally be addressed at source, by
improving/diverting the sewage effluent. Separately, The second view is that water quality deterioration may
management decisions may be taken to prevent be affected by flow modifications, but are ultimately caused
reduced flows from exacerbating the pollution by other non-flow-related activities, such as sewage
problem (see water quality box) outflows. Accordingly, environmental flow assessment
should only be used to address those issues which
• Increased sediment input to the river due to
are ultimately caused by the flow regime: sources of
catchment erosion: This should be addressed at
water quality problems should be addressed at source
source, by improving vegetation cover, and/or other
– by improving irrigation methods or sewage treatment.
anti-erosion measures. Higher flows could be used
Environmental flows, in this view, should not be set to
to transport the sediment, but this would not be
allow for the artificial dilution of effluents. Under this view, a
considered an environmental flow, it would simply
management decision may be taken to dilute effluents, but
be a management decision to allocate water for
this is not seen as part of the ‘environmental flow process’.
channel flushing
In reality, both these views have their place: it is obviously
better to improve management practices to reduce
pollutants at source. But where water quality problems
are intractable, or prohibitively expensive to deal with at
source, dilution flows may be the best option for ecosystem
protection in the immediate future. Under these latter
circumstances, flow recommendations that ignore pollution
issues can lead to recommendations with highly damaging
social and environmental impacts.

16
Part C:
Environmental flows:
key issues

17
 PART C:

Implementation

• Implementing environmental flows is about getting water flowing down the river. This may
often require difficult decisions about reducing present uses, and depends on social and
political will, and economic priorities
• T
 here has been enormous development of environmental flow assessment methods,
but relatively little implementation.

Assessment of environmental flows is about predicting can be a powerful pressure to provide flows. Over the last
what sort of flow regime will be necessary to achieve any few years, there has been an increasing understanding
particular ecological state in a river. Implementation is among most people that over-exploitation of natural
actually getting the required water down the river at the resources is not a sensible strategy in the long-term, and
appropriate time. In any context, implementation presents this new appreciation of sustainable use of resources can
an immeasurably greater challenge than assessing the be harnessed to lobby for environmental flows. In rivers
necessary flows. Conservationists need to ensure that that are not yet fully allocated or over-allocated (like the
they do not devote a disproportionate amount of effort to Mara River in Kenya and Tanzania), environmental flows
discussion and debate over the appropriate environmental don’t have to be implemented – they are already there,
flow methodology, neglecting the more important effort of and water allocation plans must then factor in the need to
working for implementation. reserve enough water to meet the environmental needs.
This may be politically more achievable.
Throughout the countries in the world that have set
policies for environmental flows, it’s probably fair to say Further information on measures that can help to
that the assessment of flow requirements has always implement environmental flows is set out in ‘Allocating
run far ahead of the implementation. Especially in over- Scarce Water’ (WWF 2007).
allocated rivers, this requires difficult decisions and
actions, such as reducing water use, or building additional
storage. In addition to significant problems of political will
or finance to implement environmental flows, there can
also be practical problems. For example, dam outlets may
not have the capacity to deliver necessary flow volumes.

The consequences of this mismatch between ambition

and reality can be seen in the fact that several hundreds
of environmental flow assessments have been
undertaken, compared with only a handful of successful
implementations world-wide.

Strategies and actions to implement environmental flows

will always be very case specific – there’s no recipe for
making them happen. If the river is over-allocated then
it will require regulations or compensation to persuade
users to reduce water use. Often the pressure from
downstream users (particularly in trans-boundary rivers)

18
 © P HARRISON / STILL PICTURES
Lesotho. Lesotho has abundant supplies of freshwater, some of which is stored and transferred to neigbouring South Africa.

A case study of environmental flow implementation

The Lesotho Highlands water project, Lesotho and South Africa

(Summarised from the IUCN Water and Nature •  ssumption: Removal of 95% of the flow would
A
Initiative Report) have little impact on the downstream rivers other
than close to the dams. The environmental flow
The Lesotho Highlands Water Project was established by
assessment showed there would be significant
Treaty in 1986 between the governments of Lesotho and
hydrological, biophysical and socio-economic
Southern Africa, to transfer water from Lesotho to South
impacts all the way to the Lesotho border.
Africa and generate hydropower for Lesotho. Twenty-
five specialists from 23 disciplines took part in the •  ssumption: People downstream of the proposed
A
three-year environmental flow study, using the holistic Lesotho Highlands Water Project structures made
DRIFT approach. Four scenarios of likely consequences very little use of riverine and riparian resources.
of dam-induced flow and sediment changes were The environmental flow assessment showed that
produced, predicting in detail how the downstream river there were extensive and sometimes complex
ecosystems could change and how this could affect relationships between the local communities and
users of the river’s natural resources. the rivers.

Guided by the DRIFT outputs, the two governments •  ssumption: The major impacts of the dams
A
negotiated releases that were higher, more varied would be on upstream communities who lost
and closer to natural conditions than the 1986 Treaty land through inundation by the reservoirs.
required. Lessons learnt were that the environmental The EFA showed extensive existing and potential
flow assessment should have been completed in the future economic and social impacts downstream
early scoping phase of the development, to inform of the structures.
structure location, design and desirability. Several major
assumptions made in the 1970s and 1980s that guided
the content of the 1986 Treaty were shown by the
environmental flow study to be erroneous:

19
 PART C:

A scientific
and social process
• E
 nvironmental flow assessment is a combination of scientific and social elements – scientists
can do the best assessment of flow needs, but they won’t be implemented unless people
know why the flows should be left in the river, and think that it is important to do so
• T
 he most effective way of getting the science accepted is to embed the environmental
flows process in an overall basin management process, which combines the use of the
river with its protection.

Environmental flows ultimately depend on the social, inevitably represent a wide variety of viewpoints, and are
economic and political will of the stakeholders to make unlikely to come to a consensus when presented with an
them work. You can have the best hydrology, the best isolated demand to accept that a proportion of the water
ecology, the best geomorphology in the world, but if in a river will be unavailable for their consumption.
people aren’t convinced of the need for environmental
flows they are unlikely to be implemented. Of course, the An example to illustrate the difference between
better the scientific understanding, the more likely people the scientific aspects of environmental flows,
are to be convinced. This makes the process of assessing and those which involve choice and judgement
and implementing environmental flows an interesting
In terms of the environmental flows process, the important
combination of science and societal judgement.
prerequisite is for the practitioners to recognise which
Because it was developed as an eco-hydrological aspects of that process are scientific and which aren’t.
process in the 1970’s and 80’s, it took some years for the The box below provides a simple illustration:
early proponents of environmental flows to realise this.
• The question ‘How many and what species of fish are
There was a gradual realisation in the 1980’s that there
there in a river?’ is open to a scientific investigation –
needed to be a social component to the process – that
zone the river and quantify the habitats, get the nets
the stakeholders needed to have a say in the uses and
and the electro-fisher, go and sample, calculate the
consequent condition of the resource. But, it wasn’t until
catch per unit effort, do mark-recapture estimates,
the 1990’s and the new millennium that there has been
model the densities etc. The results may be accurate
a full realisation that environmental flow assessment is a
or inaccurate, but this is a question that a competent
social process with an eco-hydrological core, rather than
fish biologist can tackle with confidence.
an eco-hydrological process with a social add-on.
• The question ‘How many and what sort of fish
Setting environmental flows in an isolated scientific
should be in the river?’ may be informed by science,
process is unlikely to be successful, because the crucial
but is in the end a question of societal judgement –
key to acceptance is the understanding and buy-in of
does society want to use all the water for irrigated
the stakeholders and those who will need to implement
agriculture, so there will be no fish; or, do they want
any changes. Stakeholder groups may include local
to protect the river as a national park, to maximise
communities, municipalities, industries reliant on water,
the biodiversity? More often, the answer will be
agriculture, local and national government agencies,
somewhere in between – we want some irrigation,
international NGO’s, scientific institutions, nature
but also to keep the river in a healthy state for fish and
conservation bodies and others. These groups will
people downstream. The final setting of environmental
flows is a decision based on these values, and without
that decision, there is no basis for setting any flows.

20
 PART C:

Choosing a suitable
assessment method
• T
 here are several methods for assessing environmental flows. Some are very quick modelling
or extrapolation methods, requiring no or minimal extra work. Some require years of fieldwork
and specialists from a number of disciplines
• The choice of method will depend on: the urgency of the problem; the resources available for the
analysis; the importance of the river; the difficulty of implementation; the complexity of the system
• In some cases, rather than conducting an expensive analysis, it will be best simply to let some
water down the river and see if it achieves the objectives, then monitor and adjust the flows
accordingly.

There is no one method or approach for assessing •  ow urgent is it to get the flows down the river? If the
H
environmental flows. The 200 different methods that river is relatively unexploited, and the assessment is
have been developed globally seem an awful lot, but, being done for future planning and allocation, it may
apart from reflecting the endless ingenuity of water be worthwhile spending some time to collect the
scientists, this variey of methods is also a consequence relevant data before any decision is made. On the
of the endless variety of water resources and their other hand, if the river has been reduced to stagnant
socio-economic contexts. These range from wild and pools due to over-abstraction upstream, then getting
scenic rivers in developed countries, with abundant some water flowing immediately might be the best
public concern and money for environmental issues, to option, doing the detailed work to figure out the right
degraded and polluted urban rivers in third world slums. amounts and timings later.

Assessments can take several years, and cost millions • What resources are available? If you don’t have
of dollars, or they can be done in a matter of hours reliable hydrological information, then you won’t
using extrapolation models. Generally, you can have know how much water is available. Under these
a high-confidence, very explanatory, easily-defensible circumstances there is no point in elaborate studies
assessment if you spend the time and money; or you can of the ecology of the system, because the flow
have a quick and easy, inexpensive, lower-confidence assessment will be highly uncertain. Similarly, if you
estimate that may need to be monitored and revised. are using scientists unfamiliar with the assessment
You can also start with a quick and easy method, and process, or you haven’t got or can’t afford highly
simply move on to a more comprehensive assessment if trained specialists, then there’s no point in trying to
anyone challenges it. There is no one right way to assess use complex procedures.
environmental flows, and the context is everything.
• How important is the river? Where there are
How should one therefore go about choosing the particularly important biodiversity sites, or socio-
appropriate method? Deciding on the type, complexity economic issues, it may be sensible to conduct
and duration of the assessment depends on a lot a more rigorous assessment.
of variables.

21
 PART C:
Choosing a suitable assessment method

•  ow difficult will it be to implement the environmental

H The adaptive approach
flows? In the face of management or stakeholders
Instead of starting out by conducting a predictive
who are dubious and critical of the value of
assessment of environmental water needs, the adaptive
environmental flows, it may be necessary to spend
approach instead proposes that some water is allowed
time and money to build a defensible case for the
to flow down the river, accompanied by an effective
assessment. If this isn’t done, the recommendations
monitoring operation that allows for the response of
won’t be accepted. But, if everyone agrees in
the river to the flows to be assessed. The information that
principle, the best kind of assessment is to get some
is developed in this way will be real information about the
flow down the river and monitor to see if it is achieving
impact of flows on the river, more valuable than the more
the desired objectives
uncertain information yielded by any modelling exercise.
•  re complex ecosystems such as deltas or wetlands
A
An adaptive management approach may prove to be a
involved? Some of the quicker and more basic
quicker and more accurate method of assessing flow
methods for assessing environmental flows are
needs than extended assessment methods. ‘Adaptive’
likely to be more adequate for assessing flow needs
management approaches may be particularly appropriate
in simple river channels. However, more complex
in contexts of extreme water stress, where over-exploited
freshwater ecosystems such as wetlands and deltas
rivers run dry for some or all of the year. Under these
typically have more complex environmental water
conditions of dry or nearly-dry rivers, it does not require
needs, for example pulses of water at particular times
much insight to understand that insufficient water is being
of year to inundate wetlands or maintain delta channel
allocated to environmental needs. Spending extended
structure. Under these circumstances, a more
periods of time looking for some kind of certainty over
sophisticated analysis may be required.
the precise ecosystem needs for water, while the river
continues to run dry, is clearly not in the best interests
of the river, and may not be the most appropriate way to
increase understanding of the environmental water needs
of the river.

The philosophy behind the adaptive approach is important

even where extensive environmental flow assessment
methods have been used. It’s always important to
continue to monitor the state of the river, and if necessary
adjust environmental flow allocations. Of course, such an
approach will be a necessary part of any climate change
adaptation strategy.

22
 PART C:

Setting objectives

• T
 here’s no one correct set of environmental flows for a river – the answer will depend
on what people want the river to do for them
• D
 ifferent sorts of rivers will have different requirements. For example, more natural flows
may be required for a river in a protected area than in an urban or industrial catchment.

There is no single ‘correct’ environmental flow for any The graphs in Figure 2 show hypothetical time series of
given river. Removing water from rivers is always likely to flows in rivers over one year, and differing objectives that
have some impact on the ecology of rivers. The question have been set for differing conditions. Natural flows
is how much impact is acceptable, and what objectives (in light blue) compared to environmental flows (dark blue)
are we trying to achieve with management of the river? designed to maintain elements of the dry and wet season
Much environmental legislation globally incorporates base flows, and occasional floods, while allowing a
classification systems that recognise that society will proportion of the flow (between the lines) to be harvested
wish to conserve some rivers to a higher quality than for off-stream uses such as domestic and industrial
others.2 For example, it is likely that we will want to supply, power generation, or irrigation.
conserve rivers that run through national parks, or with
particularly important fisheries, to a higher level than rivers
in extensively developed urban areas. In other contexts,
there may be vital hydrological processes that we need to
conserve – commonly, flows may be necessary to prevent
saline intrusion into farmland and groundwater supplies
at the mouth of rivers (e.g. Yangtze River in China), or to
maintain the structure of important delta ecosystems
(e.g. Indus River in Pakistan).

Setting the objectives for the management of the river

is therefore a vital requirement for any assessment of
environmental flow needs. Some environmental flow
methodologies provide a menu of the environmental
consequences of different levels of flow modification,
but ultimately there has to be a choice: what level of
flows, for what purposes?

A common challenge is how to express these

consequences and objectives in terms which are
comprehensible to people who may be unfamiliar
with environmental science, ecosystem functioning,
hydrology, and all the other ologies involved in flows
for the environment. One solution that has proved very
successful is the ‘Objectives Hierarchy’ [see Annex A].
2 These management classes and objectives are variously known as the Desired Future State (DFS) or more recently as the desired Ecological Management Class (EMC),

23
 PART C:
Setting objectives

Figure 2 Differing objectives for different rivers.

a) River flowing through a conservation area c) A spring-fed river (less variable flow)

Natural flows Natural flows

Environmental flows Environmental flows
Flow

Flow

One year One year

A) For the river flowing through a conservation area the C) For a constantly-flowing spring-fed river, the
emphasis is on maintaining as much of the natural flow emphasis is more on maintaining strong base flows,
regime as possible, so as to minimise biodiversity loss and less on floods.
due to the effects of human water use.

b) River flowing through an urban area d) A temporary river (no flow in the dry season)

Natural flows Natural flows

Environmental flows Environmental flows
Flow

Flow

One year One year

B) For the urban river, the aim might be simply to maintain D) The opposite is true of a flashy seasonal river, in which
reasonable water quality and some recreational fishing, the biodiversity depends more on sporadic high flow
so the base flows can be reduced and less floods are events than on the lower flows.
needed.

24
 © M HARVEY / WWF-Canon
Wildebeast crossing the Mara River, Masai Mara National Reserve. Kenya. Flows in the Mara River through the Reserve are
threatened by upstream abstration.

Environmental flows for a river flowing through protected areas

The Mara River, Kenya and Tanzania

The Mara River rises in the Mau escarpment of Kenya Masai Mara Game Reserve, and one on the Kenya/
and flows south through agricultural land, into the Masai Tanzania border in the protected area. The specialists
Mara Game Reserve, from which it crosses the border agreed on draft objectives to maintain the river in a
into the Serengeti National Park in Tanzania, turns near-natural state, and recommended flows were
westward out of the Park, flows through the extensive set out for base and flood flows, both in normal and
Mara wetlands, and then into Lake Victoria. It is the only dry years. Extrapolating over the long term, these
constantly–flowing river of the Serengeti ecosystem, and recommendations would amount to an annual average
is essential to the functioning of the massive annual wild of just over 50% of present flows.
animal migration – 1.5 million wildebeest and another
The Mara example illustrates the great advantage of
million zebra, antelope and associated predators.
assessing environmental flows before the river has
Forestry clearance, and irrigated agriculture in the upper
already become over-abstracted. Far from requiring
catchment outside the protected areas, threaten dry
a decrease in water abstractions, the project has
season flows in the Mara, and the consequences of flow
demonstrated that there is some water available for
reduction will be severe – during the intense drought of
further development: the objectives of the protected
1993, nearly 400,000 wildebeests perished, and if the
areas can be met with just over half of the total water
river stops flowing during the dry season, the migration
flowing down the river in average years. The other
could be permanently disrupted.
outcome of the assessment is to pin-point that water
In 2003, WWF initiated an Integrated River Basin shortages will only arise, under the present abstraction
Management project to address the deteriorating regime, during the dry season of particularly dry years.
environmental conditions of the basin. A major part of This happened in the drought of 2005, when irrigation
this project has been to train a group of Kenyan and demands, and the faster run-off from deforested areas,
Tanzanian specialists in environmental flow techniques, reduced the base flows to less that 0.5 m3s-1 in the
with help of the Global Water for Sustainability protected areas. Stakeholders and government agencies
Programme (GLOWS) and UNESCO-IHE. In October can now concentrate on ways to mitigate the effects of
2007 an initial assessment of environmental flows was drought, (for example storage in the upper catchment;
made using the Building Block holistic Methodology on-farm storage for irrigation, or compensation for
(BBM) for 3 sites in the river – 2 upstream of the reduced irrigation during droughts).

25
 PART C:

Policy and legislation

on environmental flow
assessment 3

• Many countries now have legal requirements for environmental flows

• B
 ut, legal protection of environmental flows represents an anomaly in the field environmental
protection policy and legislation: it’s the only aspect of environmental assessment in which the
onus is on to making a case for protection of the resource, rather than the potential user making
a case that use of the resource is the best option
• It may be effective to confine laws to the principles, and set detailed requirements in policy,
which can be more flexibly applied.

Too often, environmental flows are in an invidious position what this means or how managers should measure
compared to many other aspects of environmental whether they are achieving this. Similarly, the US Water
assessment and protection – it is the only one in which Pollution Control (Clean Water) Act of 1972 requires water
the environment is expected to justify its own protection. managers to ‘evaluate, maintain and restore the chemical,
This unique concept probably arose by default, as a physical and biological integrity of the nation’s waters’, but
result of the dam-building bonanza of the 1950’s and without a quantifiable methodology, managers are often
60’s. At that time, there was a general perception that at a loss to know whether or not they are in contravention
any freshwater ‘escaping’ to the sea down a river would of the law. The EU Water Framework Directive makes
be wasted, and that every drop should be intercepted an attempt to define the limits of its classes ‘high, good,
if possible. The concept of environmental flows was moderate, and poor’, but ultimately the definition has to
developed to make a case for some water to be left be developed on a water-body by water-body basis, in
in rivers. relation to the natural range of conditions for each.

Now, most countries have moved on to a recognition of

the need to look after their natural resources, and many
countries have an explicit requirement for environmental
flows as part of their national, provincial or state
strategies. Many others have an implicit requirement –
prominent among these are the countries of the European
Union, for which the EU Water Framework Directive
requires the achievement of ‘good’ ecological conditions
for all water bodies by 2015, but no specific requirement
for environmental flows.

Such legislation rarely provides careful instructions on

how the environmental flows are to be calculated or
achieved. The pioneering South African Water Act of 1998
requires water to be ‘reserved’ for basic human needs
and for the environment, and for a classification system to
be developed. The Act and its associated parliamentary
white papers mention the need for ‘sustainability’,
‘sustainable management’, ‘sustainable ecosystems’
etc. 14 times, but without any operational definition of

3 For a more extended discussion of the policy aspects of environmental flows and water allocation, see the WWF publication ‘Allocating Scarce Water’ (2007).

26
 © C MARAIS / WWF-Canon
Orange River, South Africa. South Africa’s 1998 Water Act is one of the most ambitious attempts undertaken to protect and restore
environmental flows.

Lessons from South Africa’s Water Act

South Africa’s 1998 Water Act provides one of the Secondly, even sophisticated flow assessments don’t
world’s most progressive attempts to address social and stop the arguments raging. Many water managers
environmental needs for water. The Act requires that a in South Africa now consider that the assessment
social and environmental ‘reserve’ of water be assessed, methods currently in use are over-complicated and too
recognised and protected; once this has been done, the expensive. However, the original directive from South
remaining water can be allocated among users. Africa’s Department of Water Affairs and Forestry was
for “methods that are scientifically acceptable and legally
Despite the progressive legislation, there have been
defensible”. The rule of thumb is that you can have
significant teething problems, with important lessons
complex and expensive methods that are scientifically
learned. Firstly, the Act requires that the environmental
acceptable and legally defensible, or you can have quick
‘reserve’ (flow allocation) be accepted by the Minister
and inexpensive methods that allow for implementation,
once assessed. It then becomes a legal obligation. While
but require on-going monitoring and may be hard to
this provides strong protection for environmental flows, it
defend if subject to robust legal challenge. You can’t
provides a deterrent to implementation, and can militate
have both.
against an ‘adaptive’ approach. Hence, there may be
a balance between the strength of legal recognition of
a particular flow requirement, and implementation.

27
 Part D:
Assessing
environmental flows

28
 PART D:

Methods for the assessment

of environmental flows
• T
 he 200 EFA methods can be grouped into five commonly-used generic types which require
variable time and resources, from instant look-up tables to multi-year field investigations
• In addition, there are two approaches which could be highly effective and most appropriate
in some situations:
• L
 etting down some water flow downstream, without a prior assessment, and monitoring
to see the effects
• C
 hanging the onus to the potential user, who is required to show that the use will not
unacceptably degrade the river.

Because rivers are so diverse, and the ecological, social, This table, and other similar approaches are typically
cultural and economic contexts are also so different for based on the results of a number of detailed assessments.
each case, there has been a proliferation of methodologies
As a general rule of thumb, these approaches suggest
for the assessment of environmental flows. There are now
that about 50 – 70% of the annual flow is needed to keep
over 200 different ways to come up with the flows required
the river in an excellent environmental condition, about 20
to maintain riverine ecosystems. The different approaches
– 50% for reasonable conditions, and 10 – 20% will keep it
can be grouped into five generic types, described below,
in a poor condition – impoverished but still flowing. Taking
with two additional alternative approaches.
out more than 90% of the flow will more or less guarantee
that the river and its biodiversity is seriously damaged or
THE FIVE MAIN APPROACHES
destroyed. The precise amount, and very importantly, the
Hydrology-based/look-up table approaches seasonal and yearly patterns of flow, will depend on what
sort of river, where it is, and what lives in it.4
This is the original, and simplest, of the types. Hydrology-
based methods are confined to the use of existing, or
modeled flow data, on the assumption that maintaining
Figure 3 The Montana Look-up table of Tennant (1976)
some percentage of the natural flow will provide for the
environmental issues of interest. An example is the “Montana Method” for prescribing Instream Flow Regimens for
Range of Variability Approach of Richter et al (1997), which Fish, Wildlife, Recreation and Related Environmental Resources.
describes the natural range of hydrological variation using Narrative Description Recommended Base Flow Regimens
32 different hydrological indices, which together describe of Flows /1 Oct – Mar : Apr – Sept
the magnitude, timing, duration, frequency and rate of
Flushing or Max 200% of the average flow
change that characterize the flow regime of the study
Optimum Range 60% – 100% of the average flow
river. Flow management targets are then set as ranges
Outstanding 40% 60%
of variation for each parameter.
Excellent 30% 50%
Look-up tables are typified by the Montana Approach Good 20% 40%
(see Figure 3), perhaps the original of all the methods, Fair or Degrading 10% 30%
developed by Tennant in 1976. This provides a table which Poor or Minimum 10% 10%
indicates the % of the average (natural) flow required in the Severe Degradation 10% of average flow to 0 flow
wet and dry season, to maintain conditions described as:
1/ Most appropriate description of the streamflow for all the
“Optimum (60 to 100%), outstanding, excellent, good, fair
parameters in the title.
or degrading, poor or minimal, and severe degradation
(less than 10%)”.

4 Most importantly, more than 50 – 70% of the dry season flow is likely to be required to maintain excellent environmental conditions.

29
 PART D:
Methods for the assessment of environmental flows

Extrapolation approach Hydraulic rating methodologies

This approach uses results of existing studies to model These methods measure changes in the hydraulic habitat
a relationship between flow levels and environmental available (wetted perimeter, depth, velocity etc.) based
outcomes. This method was developed in South Africa, on a single cross-section of the river that measures the
using the results of a large number of detailed studies, shape of the channel. This cross-section is used as a
and correlating the results to: the level of environmental surrogate for biological habitat, and allows for a rough
objective (A = natural, to D = largely modified); a regional assessment of changes to that habitat with changing
characterization of the hydrology; and a hydrological index flows. The required flows can be inferred from an
which generally describes the reliability or ‘flashiness’ of assessment of the habitat available for indicator species.
the flow regime. The method provides a recommended
percentage of the natural flow, and this is broken down Habitat simulation methodologies
into monthly duration curves for normal and drought years. These are a development of the hydraulic rating
This method is only possible for regions in which methodologies. With these methods, multiple rated
numerous existing environmental flow assessments have cross-sections are used in a hydraulic model to simulate
been done using more comprehensive methods, the conditions in a river reach, again based on wetted
to provide the data set for the extrapolation. perimeter, depth and velocity. Biological sampling of
indicator species, combined with measurements of the
hydraulic characteristics where they are caught, are used
to populate the habitat part of the model.
Figure 4 Flow Building Blocks of the type used in the Building Block Methodology (BBM). The blocks are basically those elements of the flow
variability that are considered to have particular ecological functions

Building Block 2:
Building Block 3: Wet season base flow
Spawning flood
FLOW LEVELS

Building Block 1:
Building Block 4:
Dry season base flows
Annual flood

JAN FEB MAR APR MAY JUNE JULY AUG SEPT OCT NOV DEC

30
PART D:
Methods for the assessment of environmental flows

The combined hydraulic/biological model then calculates availability, and have the credibility of the joint expertise of
the area of preferred habitat available for the indicator a number of specialists in different fields.
species at different flows, and can be used to infer the
ALTERNATIVE APPROACHES
required flows.
Adaptive approach or ‘see what happens’ method
This method, and particularly the Instream Flow
Incremental Methodology (IFIM), has been used The ‘adaptive approach’ is not so much an assessment
extensively, especially in the United States, and flow method, more of an action plan. This approach is to let
recommendations based on it have been successfully water down the river, and monitor the results to see if
defended in court. they meet objectives. This has the advantage that it does
not require any sophisticated predictions of the effects of
Holistic methodologies
flow, and can provide feedback based on real experience.
These are based on the use of multiple specialists However, it does require that either some form of storage
in different fields to provide a consensus view of is available, from which the experimental flows can be
the appropriate flows to meet a pre-defined set released, or that users are prepared to forego allocated
of environmental objectives, or to describe the water in order that it be allowed downstream. There also
consequences of different levels of modification to needs to be a willingness to release the flows without
the flow regime. Most of these methods make use any more detailed justification, often problematic in
of a hydrologist and a hydraulics engineer to provide contested environments.
the baseline data on flows and hydraulic conditions,
‘Upside down’ approach
freshwater biologists for fish, invertebrates, and riparian
vegetation to characterize the requirements of the biotic In this approach, the burden of proof is reversed, and
communities, a geomorphologist to predict the changes the user or potential user is required to demonstrate
in sediment transport and channel maintenance at that the proposed use of the river’s resources will not
different flows, a water quality specialist, and a socio- unacceptably degrade the resource, or ‘impair the public
economist. A number of different specific methodologies trust’, in the legal sense in the United States, where this
exist (eg Building Block Methodology, DRIFT), and these approach has been pioneered. This would probably be
provide structured frameworks for the collection, analysis the long-term goal for the protection of water resources,
and integration of the data to provide an expert prediction since it is aligned with the other Environmental Impact
of the effects of flow modifications. Assessment (EIA) methods, in which the onus is on
the potential user to demonstrate that the proposed
This group of methodologies has become very widely
development is not unacceptably damaging.
used over the past decade, since they are robust, can
be used with different objectives and levels of data

31
 © F X PELLETIER / WWF-Canon
The Indus River, Pakistan. Low flows have had devastating impacts on the river.

Assessing environmental flows for people’s livelihoods. The Lower Indus River, Pakistan

The Indus River originates on the Tibetan Plateau and Following the 1994 Water Accord apportioning available
flows 3,000 km to its delta in the Arabian Sea. It is the waters in the river between the provinces of Pakistan,
primary source of water for Pakistan and its irrigation – an environmental flow assessment was undertaken
dependent agricultural economy. The Indus delta covers for the river, in particular focused on the water needs
an area of some 5,000 km2, of which 2,000 km2 is a of the delta. An initial recommendation has been
protected area. The fan-shaped delta is the sixth largest made for a continuous base flow of 140 m3s-1) together
in the world and supports a population of over 130,000 with intermittent floods amounting to just over
people, whose livelihoods are directly or indirectly 30,000 million m3 in 5 years, sufficient to maintain delta
dependent on flow down the Indus River. channel structure. This flow is significantly less than 10%
of the naturalized flow to the delta. This assessment
As a result of abstraction for irigration the amount
has been reviewed by an international panel, and some
of water in the Lower Indus River has decreased
shortcomings were identified. Concerns were also raised
dramatically from around 85,000 million m3 per annum
about the transparency of the process.
in 1892 to 12,300 million m3 per annum in the 1990s.
In some years, the lower river has almost completely Significant controversy has followed the conclusions
ceased flowing. The reduction in freshwater inflow has of the study, and the recommendations are yet to
led to severe encroachment of the sea into the Delta be implemented. Many stakeholders have legitimate
area. Saline water has intruded 64 km inland and half a concerns over using a flawed study as the basis
million hectares of farmland have thus far been lost, and for longer term management decisions for the river.
water supplies contaminated. Furthermore, the abundant However, despite the limitations of the study, in a
freshwater discharges and nutrient-rich sediment situation in which there is presently virtually no flow in
load historically supported a highly productive coastal the lower river for extended periods, it may be best to
ecosystem, including mangrove forests and fisheries, accept and implement the recommendations as an initial
on which local communities depended for their option – it’s better to have some water in the river rather
livelihood. The decline in freshwater flows to the delta than none. At the same time, it will be very important to
has led to a general reduction in the health of the monitor the results of the recommended flows, to see
floodplain and delta ecosystems. whether they achieve the objectives set, and to revisit
the assessment, in the light of improved knowledge of
the system. This would be an example of the good use
of an adaptive, ‘see what happens’ approach.

32
 PART D:

Indicators
• It would seldom be possible to measure all the aspects throughout all the river before
assessing the environmental flows. So, most methods rely on flow indicators at selected sites
to give clues to the flow characteristics required to maintain the desired characteristics
• T
 hese indicators should ideally be those which react strongly to flow changes, and which
can be investigated to measure their responses to different flow velocities, volumes, depths
and/or habitat availability
• T
 ypically, representatives of the following groups are used: fish; benthic (bottom-dwelling)
invertebrates; riparian (river-bank) vegetation; water quality; sediment transport;
socio-cultural indicators.

Environmental flows are intended to ensure some Fish

predetermined ecological stare for a river. People have to
These are usually the best known of the biota in the
decide what that ecological state should be in relation to
river, and are also of interest to lay people, because
the goods and services that they value and that the river
of fishing. They provide a variety of lifestyles, and life-
provides naturally. This means maintaining all the aspects
stages (breeding, egg-laying, juveniles, and adults) all
of the riverine ecosystem that go to make up a balanced
requiring different habitats and conditions. They normally
sustainable system: the biodiversity; the water quality;
require at least some water (in pools), some base flow to
the sediment dynamics; the microbial processes. But, we
inundate habitats, some higher flows (to initiate breeding
can’t measure all those things unless we have unlimited
migrations), and, in the case of floodplain dwellers or
time and resources. So we use indicators which we judge
breeders, floods to lift them onto the floodplain and fill
will provide the best reflection of the ecosystem as a
the wetlands. By sampling the selected fish species, and
whole. Because it is flow that we are primarily interested
characterising their habitat preferences, it is possible to
in managing, this means the indicators should be those
predict the flows that will maintain these habitats. If a
species and components that are most sensitive to flow
particular flow provides abundant habitat for the most
conditions, either in terms of the current velocity, the
sensitive species, then we can assume that less sensitive
depth, the width of the river, the frequency of flooding etc.
species will also be catered for.
The most commonly used indicators are the fish,
the benthic (bottom-dwelling) invertebrates (insects, Invertebrates
crustaceans, worms, and snails), the riparian vegetation,
These are the most diverse group of animals in the
the water chemistry, and the sediments.
river, but they are small and difficult to identify unless
Using a variety of indicators, the group of environmental you’ve been trained. Like the fish, different groups
flow specialists can build up a picture of the flow inhabit different habitats, and the hydraulic conditions
requirements for the river, from the low flows to the large can be characterised in the same way as for fish. But
floods. Of course, many other indicators may be used, invertebrates at least are much easier to catch – put a
if the resources and expertise are available. Birds and net in the current, stand upstream and kick the substrate
amphibians (frogs and toads) are particularly useful in down into the net. They are usually there in hundreds
floodplains and wetlands; algae, bacteria and fungi can per sample, and as tens of different species, so the
add important dimensions to the understanding of all information content of a sample can be considerable. The
flow requirements. invertebrates are, like the fish, most useful for indicating
the base flow requirements – the low flows that keep the
water tumbling over the rapids and seeping in amongst
the marginal roots and plants that provide refuge habitat.

33
 PART D:
Indicators

Riparian vegetation Sediments

These are the plants that inhabit the river banks. Rivers Erosion is a natural process, but is often accelerated
can usually be identified from the air by the bright green by human land-uses. Sediments are transported down
highway that snakes through the landscape, indicating the river, flushed out from some areas and deposited
the plants that keep their roots in the groundwater in others, depending on the flows and type of soils.
that seeps from the river. These plants often grow in These processes build or erode the river banks, which
characteristic bands or zones, at varying distances from are then stabilised by the vegetation. So the fluvial
the river: the reeds, sedges and rushes that live in the geomorphologist (a specialist in sediment transport in
water’s edge; the shrubs that sprout a little way from the rivers) has to analyse the sediments in the river, and
water; and the larger trees that grow further up the banks, judge the effects (net erosion or sedimentation) that will
the roots of which help to stabilise those banks. This result from different flow regimes. One thing is sure – a
succession is an indication of the availability of water and modification of the flow regime will result in a change in
the frequency with which floods reach to different levels. the sediment transport dynamics, so the channel will
An experienced botanist can tell by the distribution of the change in the long term. The geomorphologist has to
species and their seedlings, how frequent and how large determine how rapid and extensive those changes will be,
the floods should be, and how much base flow is needed and set flows that will keep the river channel forms in a
to keep the roots wet. state that will continue to provide the habitats needed.

Water quality/ chemistry Socio-cultural indicators

Water chemistry is affected by the geology and soils of In some parts of the world, rivers can have a range
the catchment, by temperature, by the flow which dilutes of important social, cultural and religious values. For
or concentrates the elements, and of course by human example, in South Asia, rivers play a vital role in a range
activities. The natural chemistry changes down the river, of religious ceremonies, and this may require the river to
as salts accumulate and nutrients flow in. A river also has have certain flows at certain times of year.
its natural self-purification capacity, as turbulent flows
aerate the water, and microbial and algal processes take
up the nutrients. So the job of the water chemist in an
environmental flow assessment is to characterise the
natural range of concentrations in the river, and to analyse
how these concentrations are likely to change as the flows
are modified.

34
 © S BLACK / WWF-Canon
The ecology of the spectacular Kafue flats in Zambia depends on seasonal flood inundation, which also supports aquifer recharge
and grazing for local pastoralists. Since the construction of major hydropower dams on the Kafue River, this vital flood is dependent
on the operation of these dams.

© M HARVEY / WWF-Canon

35
 ANNEX A:
Objectives hierarchy
– a method for setting measurable indicators to achieve a vision

One process which has been found to be very effective An example of an objectives hierarchy
in setting objectives for environmental flows (and other
Vision:
environmental issues) is the objectives hierarchy. One of
the great strengths of the process is that it connects the “To maintain as much of the natural biodiversity as
(often vague) environmental aspirations of non-specialist possible, while supplying the needs of people, and
stakeholders with the (often highly technical) requirements minimising health risks”.
of the scientists, and therefore allows both groups to High level objective:
connect in the objective setting process. An objectives
hierarchy starts with a generic vision for the river, which “To provide all homes in the catchment with piped clean
can be expressed by the stakeholders, and which is water, without compromising downstream
generally inspiring but unmeasurable. water availability, environmental health, or natural
goods and services”.
An example could be as follows:
Sub-objectives:
“To maintain as much of the natural biodiversity as
possible, while supplying the needs of people, and • To minimise mosquito breeding habitat
minimising health risks”. • To maintain a subsistence fishery
To be useful, this overall objective has to be unpacked • To maintain good water quality.
into its components, so that the resource managers
Measurable indicators:
can know what to manage for, and how to measure
whether they are achieving the objectives. A hierarchy of • To ensure flows greater than 1.5 m3 sec-1 at
increasingly detailed objectives can provide a link between downstream sites during the dry season
the measurable indicators, which may not be readily
• To maintain a benthic invertebrate index of >100
understood by many stakeholders, and the vision. The box
during the dry season, with Hydropsychid caddis
provides a hypothetical example, using the vision above.
larvae as indicators in all riffle samples
In the example, anyone can understand the vision and
• To maintain catch per unit effort (CPUE) at more than
the high-level objective, but there is no way of directly
2 Cichlid fish of mass >250g, per hand-line per hour
measuring compliance. The sub-objectives are still
comprehensible, and connect to the high level, • To avoid salinity increasing above 50 mS m-1 for more
but are still open to different interpretations. The than 5% of the dry season
measurable indicators are quantifiable and unequivocal, • To maintain pH between 6.8 and 8.0
but objectives dealing with Hydropsychid caddis larvae,
and Soluble Reactive Phosphate are obscure to most • To keep Soluble Reactive Phosphates below
non-specialists. The objectives hierarchy therefore, is concentrations of 1 mgl-1 for more than 95%
intended to connect the inspiring but unmeasurable of the year.
vision, to a set of measurable but obscure indicators.
Of course, there may be several levels of intermediate
objectives, and literally hundreds of indicators, which
will form the basis of a monitoring system, linked to
appropriate management responses.

36
ANNEX B:
Definitions of some of the terms used in this document

Disturbance Regime: Hydraulics:

The pattern of events in an ecosystem which that removes A topic of science and engineering dealing with the
organisms and opens up space or other resources. Such mechanical properties of liquids. Stream or river
disturbances are seen as providing the mosaic of different hydraulics deals with the properties of flowing water
conditions that allows different animals and plants to thrive in the channel – how deep it is, how fast it flows, how
in different places and at different times. The disturbance wide it will spread, and what it will do in terms of moving
regime is therefore a very important determinant of the materials down the river. For ecologists (and fishermen),
biodiversity in any ecosystem. the hydraulic conditions describe the types of habitats
in a river in which different species will be found.
Geomorphology:
Riparian zone:
The study of landforms and the processes that shape
them. Geomorphologists seek to understand why A riparian zone is the interface between land and a river.
landscapes look the way they do: They try to understand Riparian vegetation is made up of plant communities
landform history and dynamics, and predict future along the river banks, characterized by plants that use
changes through a combination of field observation, the water seeping into the banks from the river. Riparian
physical experiment, and numerical modeling. Fluvial vegetation is important in terms of biodiversity and in its
geomorphology looks at the processes that transport role in stabilising river banks. Riparian zones occur in
sediment from the catchment through the river channel, many forms including grassland, woodland, and wetlands,
the erosion and deposition that shapes the river channel and require periodic wetting from high flows and floods, to
and results in flush away sediments, and to promote seed germination
the physical form of the different habitats in rivers. and growth.

Hydrology: Wetted perimeter:

The study of the movement and distribution of water In a cross section of a river, the wetted perimeter is the
throughout the Earth. Hydrologists address both width (measured across the river bed) that is covered
the hydrologic cycle – the processes of evaporation, with water. This will enlarge with increasing flows, creating
precipitation, runoff, and groundwater infiltration; additional aquatic habitat.
and water resources – the amount and distribution

of water available.

37
 ANNEX
Further reading
C:

Acreman, M. and M.J. Dunbar (2004). Defining

environmental river flow requirements – a review.
Hydrology and Earth System Sciences, 8(5), 861-876.

IUCN, (2003). FLOW – the essentials of environmental

flows. http://www.iucn.org/themes/wani/flow/main.html

King, J.M., R.E. Tharme, and M.S. de Villiers, (2000).

Environmental Flow Assessments for Rivers: Manual for
the Building Block Methodology. Report No. TT131/00.
From: The Water Research Commission, Private Bag X03,
0031 Rietfontein, Pretoria, South Africa.

Le Quesne, T., G. Pegram and C. Van Der Heyden (2007)

Allocating Scarce Water – A WWF primer. WWF-UK.

Postel, S. and B. Richter (2005) Rivers for life:

Managing water for people and nature. Island Press,
Washington. http://www.nature.org/initiatives/freshwater/
conservationtools/art16895.html

Schofield, N., A. Burt and D. Connell, (2003).

Environmental water allocation: principles, policies
and practices. Land & Water Australia.
http://products.lwa.gov.au/products/pr040738

The Nature Conservancy (2006) Environmental flows:

Environmental perspectives on river basin management.
Special issue: developing a global environmental flows.
http://www.nature.org/initiatives/freshwater/files/ef_nl_5.
pdf

38
References used in the text

Bunn, S. E. And A. H. Arthington (2002) Basic principles

and ecological consequences of altered flow regimes for
aquatic biodiversity. Environmental Management, 30: 492-
507.

Noss, R. F. (1990), ‘Indicators for Monitoring Biodiversity:

A Hierarchical Approach,’ Conservation Biology,
4: 355-364.

Resh, V. H., A.V. Brown, A. P. Kovich, M. E. Gurtz, H.

W. Li, G. W. Minshall, S. R. Reice, A. L. Sheldon,J. B.
Wallace, and R. C. Wissmar (1988) The role of disturbance
in stream ecology. Journal of the North American
Benthological Society 7:433-455.

Richter, B. D., J.V. Baumgartner, R. Wigington, and

D. P. Braun (1997). How much water does a river need?
Freshwater Biology 37: 231-249.

South African National Water Act, No. 36 of 1998.

Government Gazette Vol.398 No.19182,
Cape Town, 26 August 1998.

Tennant, D. L. (1976) Instream flow regimens for fish,

wildlife, recreation and related environmental resources.
Fisheries, 1: 6-10.

United Nations (1992) Agenda 21, United Nations

conference on environment and development.
United Nations, Conches.

World Bank (2008), Integrating Environmental Flows into

Hydropower Dam planning, Design, and Operations,
Water Resources and Environment Technical Guidance
Note, World Bank 11 June 2008.

39
 WWF’s Freshwater Programme in numbers
100%

•
KEEPING RIVERS ALIVE
RECYCLED

10
WWF is one of 10
organisations which has
established the Alliance
for Water Stewardship to

35%
establish an international
certification programme for
water managers and users
the Freshwater Living
Planet Index (a global
measure of more
than 700 vertebrate
animals) declined by
35% between 1970
and 2007

FRONT cover image:© Wild Wonders of Europe /Peter Cairns / WWF, back cover image: © Brent Stirton / Getty Images
100 million
12 WWF has helped
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million hectares of
our global freshwater freshwater protected
priorities are the rivers and areas in the last 10
lakes in 12 critical places years
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across 5 continents
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