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The Ecological Basis of River Restoration: 1. River Ecology for Hydraulic

Engineers

Conference Paper · December 1998

DOI: 10.1061/40382(1998)69

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MEIER, C.I. 1998. The Ecological Basis of River Restoration: 1. River Ecology for
Hydraulic Engineers. In: HAYES, D.F. (editor) Engineering Approaches to Ecosystem
Restoration. American Society of Civil Engineers, Reston, Virginia, 386-391.

The Ecological Basis of River Restoration

1. River Ecology for Hydraulic Engineers

1
Claudio I. Meier , Student Member, ASCE

Abstract

Knowledge of the basic concepts of river ecology is fundamental to understanding the

nature of human impacts on river systems and articulating sound restoration strategies. This
article reviews the fundamental elements and processes that structure fluvial ecosystems:
the physical setting, living organisms, and inputs and flow of energy. It also highlights
those factors that maintain a high heterogeneity of habitats and ecological connectivity,
resulting in higher biological diversity and productivity.

Introduction

The case for river restoration should be self-evident, considering the widespread
degradation of river systems due to dams, pollution, over-exploitation of species, water
diversions, intensive land-use, channelisation and floodplain development, introduction of
exotic species, etc. Because of these and other anthropogenic changes, a large proportion of
freshwater organisms is extinct or imperilled (Karr, 1996), and the ecological, recreational,
and aesthetic value of many running waters has been reduced. As a response to this
damage, river restoration (renaturalisation, rehabilitation) plans are proposed and carried
out, in the hope of revitalising fluvial ecosystems.

However, most projects focus on particular organisms or on single physical characteristics

of streams, and have relatively narrow goals, without due regard for ecosystem-wide
processes and overall biodiversity. For example, procedures as diverse as enhancing
physical habitat for trout or stabilising river banks are routinely referred to as “river
restoration”. Even though these activities could, sometimes, be part of a successful
restoration project, they should not be confused with the concept itself.
__________________________________________

1
Assistant Professor of Civil Engineering, Universidad de Concepción, Concepción, Chile.
Currently on leave of absence at Department of Civil Engineering and Graduate Degree
Program in Ecology, Colorado State University, Fort Collins, CO 80523.

386 Meier
Thus, there is no clear agreement on what river restoration means, or what its objectives
should be. I propose that there is a serious need for defining “river restoration” based on a
holistic consideration of a river as an ecosystem.

In this article, I present fundamental concepts of river ecology that are needed to
understand the different types of impact on river ecosystems and to articulate a coherent,
ecologically-based definition of river restoration. A companion paper (Meier, 1998) deals
with these impacts and attempts to define river restoration from an ecological perspective.
Both papers are addressed to an audience of hydraulic engineers and hydrologists, because
these professionals are usually charged with managing rivers, and should also be involved
in any river restoration interdisciplinary team. Readers are referred to Jeffries and Mills
(1995) for a basic introduction to stream ecology. Allan (1995) provides an exhaustive
review of this interesting discipline.

The natural river ecosystem

A river ecosystem consists of many interacting organisms of different species (the biota)
living in a physical setting (the abiotic environment). These organisms need an energy
source (e.g., food to stay alive, grow, and reproduce) and a place to live in the physical
environment (a habitat). They also interact with each other (biotic interactions), for
example through predation (either as prey or predator) and competition (fighting for
limiting resources, such as space or food).

The physical environment

A river environment is the result of physical, chemical, and biological processes occurring
in the catchment basin over a range of time-scales. Climatic factors (mainly precipitation
and temperature régimes), acting over the basin’s geology, determine the landscape, the
character of the soil, and the type of vegetational cover, or absence thereof (Morisawa,
1985). In turn, all of these control the discharge régime (hydrology) of the river, together
with its inorganic sediment load (silt, sand, gravel, etc.), organic sediment detritus (leaves,
twigs, large woody debris, etc.) commonly referred to as particulate organic matter (POM),
dissolved matter fluxes (solutes), and stream temperature régime.

In alluvial streams, the overall channel morphology is determined by the discharge and
sediment load. In any given reach, the hydraulics of the flow, and the load entering the
reach interact with bank and bed materials and with the riparian vegetation. The resulting
morphology of the reach (width, depth, slope, channel pattern, etc.) is a balance between
erosional and depositional processes (Morisawa, 1985).

In unaltered river systems, these processes create a complex environment, which is highly
heterogeneous, both spatially and temporally. This changing mosaic of channel and
floodplain structures provides habitat for many different species of plants and animals, both
aquatic and riparian, whose life-cycles have evolved in response to the highly dynamic and
heterogeneous environment (Stanford et al., 1996).

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Because different species, and life-stages of a given species, have different environmental
requirements, any change in the physical environment or its dynamics can result in changes
in the composition of the biological community. For example, many insect species are
eradicated downstream of large reservoirs, because of deep releases that result in
unnaturally warm waters during winter. The current view (Stanford et al., 1996) is that the
community structure (the distribution and abundance of animals and plants) of flood-prone
rivers is mainly determined by physical processes, not biotic interactions.

The diversity of organisms

Streams and rivers contain a high diversity of plant, invertebrate, and vertebrate species.
Most plant diversity is in the diatoms and other microscopic algae that grow in a thin layer
around wetted surfaces exposed to sunlight, such as cobbles in the stream bed, or the stems
and leaves of aquatic plants. This growth is generically known as periphyton; it causes the
stones at the bottom of any clear-water river to be slippery. There are hundreds of species
of these microalgae in a stream (Allan and Flecker, 1993).

Aquatic macrophytes, the larger plants of running waters, can be flowering plants (such as
watercress and reeds), mosses and liverworts (bryophytes), or large algal species (e.g., mats
of filamentous algae). Headwater streams may contain only mosses, with diversity
increasing downstream, as slow water habitats become more common. If conditions are
adequate, there can be up to 20 macrophyte species in a reach (Allan and Flecker, 1993).

Invertebrate diversity is made up of aquatic insects (e.g., the mayflies, stoneflies, and
caddisflies dear to the fly fisherman), various worms, crustaceans (such as scuds), clams,
and snails. Minute worms and midges usually make up most of the many hundreds, or even
thousands of invertebrate species that one can find in a stream reach. Most of these species
live on or in the stream bed, and are thus generically known as the benthos.

There are three main food categories for freshwater invertebrates: detritus (particulate
organic matter, or POM), periphyton, and prey (other invertebrates or fish fry). Most
species have evolved feeding adaptations to exploit only one type of food resource, and can
thus be classified into functional feeding groups: shredders feed on coarse detritus (CPOM,
which is mainly vegetal litter imported from the riparian zone); collectors feed on fine
POM (arbitrarily defined as organic particles less than 1 mm in size), either by filtering the
water (as clams and net-building insect larvae do) or gathering from surface deposits;
scrapers, also called grazers, feed on periphyton; predators such as dragonfly larvae are
carnivores that hunt for their prey.

Classifying species into functional feeding groups can help in predicting the impacts of
alterations, because most anthropogenic changes in rivers alter the dynamics of food
resource inputs in relatively predictable ways. For example, the deposition of washload
behind a dam changes a naturally turbid river into a clear-water stream, allowing for
periphyton production at the stream bottom. This increases the food source for scraping
(grazing) species, commonly resulting in a higher production of invertebrates and fish.

388 Meier
Fishes are the charismatic megafauna of freshwaters because of their economic,
recreational and aesthetic value. They are indicators of the health of a river. The majority
of fishes eat invertebrates or other fish, and are thus at the top of the stream foodweb.

Because of the nature of river networks, freshwater fishes become segregated in drainage
basins, giving rise to genetically distinct populations or races with evolutionary time. In the
case of anadromous (sea-going) fish, a river can have different runs, i.e., populations that
can be distinguished by the timing of their spawning run from the ocean. For example, the
Columbia River had three main groups of returning chinook salmon: spring, summer, and
fall-run fish, in a host of local races, adapted to each of the different tributaries. Most of
these went extinct when large dams were built on the main stem.

Many animal and plant species exhibit this type of variability in different ways. Thus,
conserving biodiversity implies far more than just saving a given species from extinction.
Sub-species and local populations with distinct adaptations must also be considered.

The energy sources

The food for all organisms is organic matter containing stored chemical energy that
ultimately comes from the sun. It can enter the aquatic ecosystem along two different
pathways. Some of it is produced in the stream itself, as periphyton, macrophytes, and
other plants convert dissolved inorganic nutrients into organic matter, thus storing sunlight.
This plant material photosynthesized in-situ is known as autochthonous organic matter, and
most of its biomass is in the periphyton layer surrounding the stream bed.

The allochthonous organic matter is material photosynthesized in the surrounding

terrestrial or riparian ecosystems, that is subsequently imported into the aquatic system.
Dead leaves, twigs, boughs, etc. represent allochthonous organic matter inputs.

The river continuum concept (see Allan, 1995, pp. 276-281), is a theory that explains
changes in energy inputs and community structure along the longitudinal direction of
temperate streams. Figure 1 shows a generalised river stem, with stream order and width
increasing from its headwaters to its mouth. The upper reaches are light-limited because of
shading by riparian vegetation, so that production in the stream is small as compared to
import of terrestrial organic matter (litter). Shredders and collectors, that feed on coarse
and fine POM, respectively, dominate the community of invertebrates.

Mid-sized rivers are usually shallow, clear, and wide enough for riparian shading to be
reduced, favouring in-situ photosynthesis. The importance of terrestrial inputs of CPOM
decreases, and now fine POM is imported from upstream reaches. As a result, invertebrate
functional groups are dominated by grazing and collecting species.

Further downstream, large rivers tend to become too deep and turbid to sustain
photosynthesis. Most organic matter is FPOM that has drifted from upstream, so that the
majority of benthic species are collectors, filtering the water or gathering from deposits.

389 Meier
Figure 1. Pictorial representation of the river continuum (from Cummins, 1975)

390 Meier
 Connections within the fluvial system

A river ecosystem encompasses much more than the wetted part of the flowing channel. It
has diffuse boundaries with the terrestrial and groundwater systems (the riparian and
hyporheic zones, respectively), and includes bars, side-arms, floodplain lakes, and all other
features created by fluvial processes within the floodplain. These channel and floodplain
features change with time. Thus, a river ecosystem can be considered to have three spatial
dimensions (longitudinal, lateral, and vertical) that are temporally variable (Stanford et al.,
1996). It is essential to maintain connectivity along these dimensions.

Summary

Biological diversity and productivity in running waters are a result of habitat diversity and
ecological connectivity. These are created and maintained by fluvial processes of erosion
and deposition, that depend on a stream’s hydrological and sediment régimes. Changes in
the dynamics of these physical processes, or loss of connectivity along any of the three
spatial dimensions of a river, will lead to changes in the aquatic community.

References

Allan J.D. 1995. “Stream Ecology: Structure and Function of Running Waters”. Chapman
and Hall, London.

Allan J.D. and A.S. Flecker. 1993. Biodiversity conservation in running waters. Bioscience
43 (1): 32 - 43.

Cummins K.W. 1975. The ecology of running waters: theory and practice. In:
“Proceedings of the Sandusky River Basin Symposium”, Baker D.B., W.B. Jackson, and
B.L. Prater, eds., International Joint Commission, Great Lakes Pollution, pp. 277-293.
Environmental Protection Agency, Washington, D.C.

Jeffries M. and D.H. Mills. 1995. “Freshwater Ecology: Principles and Applications”.
Wiley, Chichester.

Karr J.R. 1996. Ecological integrity and ecological health are not the same. In:
“Engineering within Ecological Constraints”, Schulze P.C., ed., National Academy of
Engineering, pp. 97-109. National Academy Press, Washington, D.C.

Meier C.I. 1998. The ecological basis of river restoration: 2. Defining restoration from an
ecological perspective. In: Engineering Approaches to Ecosystem Restoration (this
volume). American Society of Civil Engineers, Reston, VA, 392-397.

Morisawa M. 1985. “Rivers: Form and Process”. Longman, London.

Stanford J.A. et al. 1996. A general protocol for restoration of regulated rivers. Regulated
Rivers 12 : 391 - 413.

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