Activity 14
FIGURE 2.5: RIVER PROFILES
2.5 Refer to FIGURE 2.5 showing river profiles.
2.5.1 Define the term longitudinal profile. (1 x 1) (1)
2.5.2 Describe the shape of longitudinal profile A. (1 x 2) (2)
2.5.3 Identify an ultimate (permanent) and temporary base level of
erosion in the diagram. (2 x 1) (2)
2.5.4 How will the deposition of sediments influence the capacity of the
dam? (1 x 2) (2)
2.5.5 Describe the difference in grade between the new longitudinal
profile and the original longitudinal profile. (2 x 2) (4)
2.5.6 Explain the impact of the presence of the dam on erosion and
deposition processes. (2 x 2) (4)
Marking Guideline
2.5 Refer to FIGURE 2.5 showing river profiles.
2.5.1 Define the term longitudinal profile.
(Side) view of a river from source to mouth (1)
[CONCEPT]
2.5.2 Describe the shape of longitudinal profile A.
It has a concave shape. (2)
It is steep in the upper course, less steep in the middle course and gradual in
the lower course. (2)
[ANY ONE]
2.5.3 Identify an ultimate (permanent) and temporary base level of erosion in
the diagram.
Ultimate (Permanent) – sea (1)
Temporary – dam (1) (2 x 1)
(2)
2.5.4 How will the deposition of sediments influence the capacity of the dam?
It will decrease the capacity / reduce the volume of water (2) (1 x 2)
(2)
2.5.5 Describe the difference between the new longitudinal profile and the
original longitudinal profile.
New longitudinal profile is ungraded and the original longitudinal profile is
graded (2)
New longitudinal profile is multi-concave while the original longitudinal profile
is concave (2)
[ANY ONE COMPARISON] (2 x 2)
(4)
2.5.6 Explain the impact of the presence of the dam on erosion and deposition
processes.
EROSION
More erosion upstream of dam due to greater water volume (2)
Erosion temporary stops at the dam (2)
Rate of erosion deceases downstream because the there is less water (2)
[ANY ONE]
DEPOSITION
Deposition of silt in the dam resulted in the multi-concave profile (2)
The rate of deposition increases as the velocity of the water is reduced
downstream (2)
[ANY ONE]
[MUST REFER TO BOTH EROSION AND DEPOSITION]
Activity 15
FIGURE 1.6: CHANGING CROSS-PROFILE OF THE VALLEY ALONG A
RIVER'S COURSE
1.6 FIGURE 1.6 shows the changing cross-profile of the valley along the river's
course.
1.6.1 In which course is the source of the river? (1 x 1) (1)
1.6.2 Name TWO elements of the cross-profile that changed from the upper to
the lower course in FIGURE 1.6. (2 x 1) (2)
1.6.3 Differentiate between the fluvial processes that shaped the cross-
profiles of the upper course and lower course of the river. (2 x 2) (4)
1.6.4 Describe the reasons for the change in the shape of the cross-profile of
the middle course. (2 x 2) (4)
1.6.5 Explain why the shape of the cross-profile in the upper course of the
river will make it the most suitable place to build a dam. (2 x 2) (4)
Marking guideline
1.6 1.6.1 Upper course (1) (1 x 1) (1)
1.6.2 Width/Breadth (1)
Depth (1)
Shape (1)
Heights of interfluves change (1)
[ANY TWO. ACCEPT DESCRIPTION OR MEASUREMENTS] (2 x 1) (2)
1.6.3 In the upper course vertical/downward erosion takes place (2)
In the lower course deposition/lateral erosion takes place (2) (2 x 2) (4)
1.6.4 (Increased) lateral erosion/(decreased) downward erosion results in a river
valley widening (2)
Sheet flow down the valley slopes will result in the lowering of the slopes (2)
River slows down in the middle course causing deposition which decreases
the depth of the valley (2)
More gentle gradient in the middle course causing deposition which decreases
the depth of the valley (2)
Meandering river will widen the valley floor (2)
Where the river exits the confines of the mountain, the river valley will be wider
(2)
Greater volume of water and lower velocity will increase lateral erosion in the
middle course (2)
[ANY TWO] (2 x 2) (4)
1.6.5 In the upper course of the river the valley is narrow (2)
Cost of construction of the dam wall will be cheaper (2)
The rocky banks will make the structure stronger (2)
The dam will be deeper because of the deep valley (2)
Cooler temperatures at higher altitude, therefore less evaporation (2)
Smaller water surface area reduces evaporation rates (2)
A deeper dam will have an increased capacity (2)
Less silt in the dam as there are fewer tributaries that enter the dam (2)
Steepness of slope allows easy flow of water into a dam (2)
[ANY TWO]
Activity 16
FIGURE 1.6: RIVER GRADE AND LONGITUDINAL PROFILES OF A RIVER
Source
Original sea
level
First fall in
sea level
Second fall in
sea level
Knickpoints
Mouth
Source
Waterfall erosion will exceed deposition
until the feature retreats and disappears
Lake
Waterfall
Mouth
Eventual possible graded profile
Source
Smooth concave profile decreasing in
angle and gradient towards the mouth
Mouth
1.6 Refer to FIGURE 1.6 showing river grade and the longitudinal profiles of a
river.
1.6.1 Give a geographical term to describe the irregular shape of
longitudinal profile B. (1 x 1) (1)
1.6.2 Name a temporary base level evident in longitudinal profile B. (1 x 1) (1)
1.6.3 What evidence suggests that rejuvenation has taken place in
longitudinal profile A? (1 x 2) (2)
1.6.4 Describe, with reasons, the changes a river meander will undergo
after rejuvenation. (2 x 2) (4)
1.6.5 In a paragraph of approximately EIGHT lines, explain the processes
that assisted the graded river in profile C to have a steep gradient in
the upper course and a gradual gradient in the lower course. (4 x 2) (8)
Marking Guideline
1.6 1.6.1 Ungraded profile (1) (1 x 1) (1)
1.6.2 Lake (1)
Waterfall (1)
Knickpoint (1)
Rock outcrops (1)
[ANY ONE] (1 x 1) (1)
1.6.3 A drop in the original sea level (2)
Presence of knick points/waterfalls (2)
Ungraded profile (2)
A sudden change in gradient (2)
[ANY ONE] (1 x 2) (2)
1.6.4 Description/Change
The meander will become incised/entrenched (2)
The meander will develop steeper sides (2)
Will form a cut-off meander/ox bow lake (2)
[ANY ONE CHANGE]
Reason
The amount of energy of the river would increase (2)
The velocity of water flowing within the meander increases (2)
The rate of downward/vertical erosion in the meander will increase (2)
A stronger flow will result in the river cutting through the meander neck (2)
[ANY ONE REASON] (2 x 2) (4)
Activity 18
FIGURE 1.6: LONGITUDINAL PROFILE OF A GRADED RIVER
1.6 Study FIGURE 1.6 which shows the longitudinal profile of a graded river.
1.6.1 What is meant by the term base level of a river? (1 x 1) (1)
1.6.2 Why is the sea regarded as a permanent base level? (1 x 2) (2)
1.6.3 What evidence in FIGURE 1.6 indicates that this river is graded?
(1 x 2) (2)
1.6.4 Why will a drop in sea level change the grading of the river? (2 x 2) (4)
1.6.5 Draw a simple longitudinal profile to show the impact of the drop in
sea level, mentioned in QUESTION 1.6.4, on the shape of the
longitudinal profile. (1 x 2) (2)
1.6.6 Explain how the interaction between erosion and deposition in the
upper and lower courses of a river help to maintain a graded profile.
(2 x 2) (4)
Marking Guideline
1.6 1.6.1 It is the lowest level to which a river can erode (1)
[CONCEPT] (1 x 1) (1)
1.6.2 The land cannot erode down below sea level (2) (1 x 2) (2)
1.6.3 The profile has a smooth and concave shape (2)
No temporary base levels of erosion evident (2)
No obstructions along the river course (2)
[ANY ONE] (1 x 2) (2)
1.6.4 Due to a change in sea level, the gradient of the river becomes steeper (2)
The river profile is no longer smooth (2)
A knickpoint forms where the old sea level and now exposed sea-floor meet
(2)
Renewed energy starts to erode the new knickpoint upstream (2)
River profile becomes multi-concave/irregular (2)
A temporary base level of erosion will develop along the river course (2)
River becomes undergraded (2)
[ANY TWO] (2 x 2) (4)
1.6.5
(1 x 2) (2)
1.6.6 Erosion dominates in the upper course, removing all the obstacles (like
waterfalls and rapids) in the upper course (2)
Eroded material is deposited in the lower course (2)
The balance that exists between erosion and deposition creates a concave
profile (2) (2 x 2) (4)
Activity 19
FIGURE 1.2: RIVER SYSTEM
1.2 Study FIGURE 1.2 based on a river system.
1.2.1 Name the drainage pattern in the upper course of the river in area A.
1.2.2 Name the underlying rock structure likely to be found in area A.
1.2.3 Is area A the catchment area or the mouth of the river?
1.2.4 State the type of erosion that causes deep valleys in area A.
1.2.5 Name a natural feature that forms at a knick point (in area B).
1.2.6 In which course of the river does deposition dominate?
1.2.7 Name the landform that is likely to form from the sand deposits at
the river mouth. (7 x 1) (7)
Marking Guideline
1.2 1.2.1 Dendritic (1)
1.2.2 Homogenous geology/Uniformly horizontal (1)
Rocks of uniform resistance to erosion (1)
Horizontal sedimentary/Massive igneous (1)
[ANY ONE]
1.2.3 Catchment (1)
1.2.4 Vertical/Downward/Down cutting (1)
1.2.5 Rapid (1)
Waterfall (1)
[ANY ONE]
1.2.6 Lower Course/Reaches (1)
Old age stage (1)
Plain Stage (1)
Stage C (1)
[ANY ONE]
1.2.7 Sand islands/Sand banks (1)
Delta (1)
[ANY ONE] (7 x 1) (7)
Activity 20
2.3 Refer to the sketches showing the profile and grading of a river.
[Adapted from file:///T:/Fluvial%20Landforms.pdf]
Refer to sketch A.
2.3.1 Define the concept longitudinal profile. (1 x 2) (2)
2.3.2 State TWO characteristics of the longitudinal profile evident in
sketch A. (2 x 1) (2)
2.3.3 Does sketch A represent a graded or an ungraded river? (1 x 1) (1)
2.3.4 Give a reason for your answer to QUESTION 2.3.3. (1 x 2) (2)
Refer to sketches B and C.
2.3.5 Identify a temporary and a permanent base level of erosion in
sketch B. (2 x 1) (2)
2.3.6 Describe the processes that the river in sketches B and C would
undergo to reach a graded state. (3 x 2) (6)
Marking Guideline
2.3
2.3.1 The side view of a river from source to mouth (2) (1 x 2) (2)
2.3.2 Smooth/Graded (1)
Concave shaped (1)
Steeper in the high-lying area (1)
Gentle in the low-lying area (1)
(Accept) Permanent base level (1)
(Accept) No temporary base levels/ waterfall (1)
[ANY TWO] (2 x 1) (2)
2.3.3 Graded (1 x 1) (1)
2.3.4 Temporary base levels not evident (2)
It has a smooth concave profile (2)
[ANY ONE] (1 x 2) (2)
2.3.5 Temporary base level: waterfall (1)
Permanent base level: sea (1) (2 x 1) (2)
2.3.6 Erosion of softer layers of rock below waterfall (2)
Headward/backward erosion will cause the waterfall to migrate upstream (2)
Downward erosion deepens the plunge pool (2)
Hard layer of rock falls into plunge pool as not supported by underlying softer
rock (2)
Deposition of sediments in plunge pool and continues towards the lower
course (2)
Equilibrium between erosion and deposition will maintain a graded state (2)
[ANY THREE] (3 x 2) (6)
