Rivers And River Processes
nRIVERS AND RIVER PROCESSES
The Hydrological Cycle
RIVER is a channel which allows water to flow from one point to another point
Water is the most precious resource in our day to day lives. It is needed for domestic, industrial and
agricultural purposes. Water comes from precipitation (rain in all its forms) and most of it ends up in
rivers and streams on its way to the oceans. The various processes involved are all shown on the
diagram above.
Drainage basin
A drainage basin is an area drained by a river and its tributaries. It is also known as a catchment area.
Two different drainage basins are separated by a highland known as a watershed.
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Rivers And River Processes
Inputs of a Drainage basin
The inputs of a drainage basin include radiation and precipitation. Precipitation refers to all forms of
water reaching the earth’s surface in the form of rain, fog, mist, slit, hail, drizzle and snow.
Outputs of a drainage basin
These include evaporation, transpiration, infiltration, percolation and runoff into the seas and oceans.
These are the ways in which water leaves the drainage basin.
Processes in the drainage basin
These include interception, transpiration, evaporation, stermflow and interflow.
Drainage patterns
A drainage pattern refers to the way a river and its tributaries are arranged in the drainage basin. The
following are some of the common patterns;
a) Dendritic drainage pattern – the river and its tributaries form the shape of a tree and its
branches. The tributaries join the main stream at acute angles. The pattern develops in an area
where the rocks are of uniform structure and resistance. It is the most common type of pattern
in the world.
b) Radial/centrifugal drainage pattern – the rivers look like the spokes of a wheel. It develops on a
dome shaped mountain range with streams flowing down the sides of the dome. In Zimbabwe,
the Pungwe, Odzi, Gairezi and inyangombe all radiate from Nyanga Mountain. The Jos plateaus
in Nigeria and Mt Kenya also have radial patterns.
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Rivers And River Processes
c) Trellis drainage pattern/ rectangular pattern - occurs where the tributaries join the main
stream at right angles. The pattern develops in areas with alternating resistant and less resistant
rocks. The main stream cuts across these bands but the tributaries erode head ward into the
soft rocks forming wide valleys. The hard rock bands stand out as escarpments with short
obsequent streams flowing down the sharp steep slopes and join the subsequent streams at
right angles e.g. the Sanyati River System
d) Parallel drainage pattern – the river and its tributaries are almost parallel in their flow. This
develops in an area of parallel folds or areas of an even gradient e.g. the Gwai River developed
over an area of uniform gradient.
e) Centripetal drainage pattern/inland pattern – also looks like the spokes of a wheel. It develops
in areas where inland depressions act as collecting points or emptying points for rivers.
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Rivers And River Processes
The way in which water flows within a river channel
Water flows in two ways in a river channel;
a) Laminar Flow – in this case the water flows in layers parallel to the river bed and it occurs where
the river bed is smooth.
b) Turbulent Flow – in this case, water moves in a circular like manner because the river bed is
rough. In this flow, the energy has less energy because some energy will be used to overcome
friction.
RIVER PROCESSES
Three main processes occur in a river channel and these are;
1. Erosion – the process of earth sculpturing by running water
2. Transportation – the movement of eroded material along the channel, and
3. Deposition – the laying down of the sediment being transported by a river
River Erosion
There are five erosive processes in a river channel;
1. Hydraulic Action – occurs when the force of the flowing water manages to remove loose
materials such as gravel, silt and sand or when the force of the water dislodges material from
the sides and the bed.
2. Solution/corrosion – the solvent action of water since it has absorbed chemicals and gases to
form weak acids. These weak acids dissolve certain kinds of rocks and carry them in solution.
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Rivers And River Processes
Solution is independent of river discharge or velocity but depends on the solubility of the rocks
forming the river bed and the acidity of the water.
3. Corrasion/abrasion – occurs when material being carried by the river is dragged along the river
bed and sides dislodging more materials in the process. The action is like scraping with a
sandpaper or using an iron file
4. Attrition – rock particles carried by the river wear away each other as they collide. It results in
larger particles becoming smaller and therefore becoming easier to carry. The numerous
rounded and smooth pebbles found in rivers are a result of this process.
The combined effect of these erosive processes affects a river in three ways;
1. Headward erosion – a river is cut back at its source to increase its length.
2. Vertical erosion – a river deepens its course
3. Lateral erosion – a river widens its valley
Factors affecting the erosive power/efficiency of a river
The erosive power of a river depends on four factors;
1. River Volume/discharge – discharge is the volume of water passing through a point along in a
river valley in cubic meters per second. Usually the greater the discharge, the greater the
erosive power of a river. Wet summers which bring heavy rains are associated with the greatest
erosive power.
2. River Velocity/Speed – fast flowing streams are much more erosive than slow moving streams.
Channel velocity is affected by such factors as;
a. Steepness of slope – steeper slope more velocity
b. Shape of river valley – narrow deep valleys have greater velocity than wide shallow
channels. This is because shallow channels have greater surface area and therefore
more friction which slows down the river.
c. Smoothness or roughness of river bed – a rough channel has more friction which slows
down the river.
3. Type and amount of load carried by the river – a river erodes its bed and sides when the load is
dragged on the bed and sides. The more the load, the better the erosive power of a river. Unless
the load is in solution which does not contribute to erosion.
4. The type of rock on the river bed – hard and resistant river beds are more difficult to erode
than soft and weak river beds.
RIVER TRANSPORTATION
A river’s load refers to the materials being transported by a river and there are two types of load,
1. Solution load – which consists of soluble products of chemical weathering and this load in
transported to the sea or inland lakes.
2. Mechanical load – this consists of all kinds of solids which are transported in three ways;
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Rivers And River Processes
a. Suspension – occurs when light materials are carried within the body of the water and
are suspended away from the river bed by turbulent flow.
b. Saltation – occurs when small particles are carried in a series of short leaps as they are
carried by the river i.e. they bounce along the river bed.
c. Traction – occurs when larger particles are rolled and dragged along a river bed
NOTE: the largest load of a river is carried in suspension and the smallest portion in solution.
RIVER DEPOSITION
Deposition refers to the laying down of the river’s load due to a number of reasons which include;
1. There may be sudden input of load which overloads the river and therefore makes it incapable
of carrying its load. E.g. when there is a landslide.
2. A river may loose its energy and therefore become incapable of carrying its load due to;
a. A decrease in gradient
b. A decrease in discharge
c. When a river widens its channel and therefore there is increased friction.
d. During flooding when a river overflows its banks and this has the effect of increased
friction.
e. When a river approaches its mouth and therefore slows down
The heavier boulders and gravel are deposited first and fine silt last. River deposition is associated with
the lower course of a river or its flood plain. Here the velocity is much reduced and therefore deposition
takes place. In reality a river may drop its load at any point along its channel as long as energy is
significantly reduced.
LANDFORMS FROM RIVER ACTION
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Traditionally a river has been divided into three main parts;
1. Upper course
The main characteristics the upper course
The valley is narrow with a narrow, shallow river channel
The valleys have steep sides
The channel has a steep gradient with features such as pot holes, rapids and waterfalls
The water is mainly slow flowing as most of the rivers energy is used to overcome the
friction of the river bed & obstructions
 The load is mainly large, angular and rough




2. Middle course, and
The Main Features
 The river has a greater discharge than the upper course
 The river has a greater sediment load
 The valley has a broad, flat floor (flood plain) bordered by gentle slopes called
bluffs
 Meanders are the main feature here
3. Lower Course
The main features of the river channel and river valley are:
 It is wide and deep with a more efficient shaped channel
 It is lined with sand and mud so this results in less friction
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 There maybe islands of silt called eyots in the river and the river can split into a
number of channels - river braiding
 The river carries a large load of alluvium
 There is a wide, flat flood plain either side of the river
 There are features such as meanders, ox bow lakes and deltas
 The flood plain is made up of a thick layer of alluvium
 A line of river cliffs are found at the edge of the flood plain called bluffs
The summary of these features is shown in the table below
Upper Course
Gradient :-5m per km
Contours :- close together to
show steep slope
Processes Active :– headward
erosion and vertical erosion
Valley shape: – steep sided,
narrow and deep.
River Flow: – fast flowing with
more downward cutting.
Features :– V shaped valleys,
gorges, waterfalls or rapids,
interlocking spurs, potholes,
river capture or piracy
Middle Course
2m per km
Gentle slopes as contours are far
apart
Lateral erosion
Lower Course
0.2m per km
Almost non existent contours to
show almost flat slope
Deposition
U shapes valleys which are much
wider due to lateral erosion
Slower flow and river begins to
meander and some deposition
takes place on the inside bends
River cliffs, slip off slopes, river
bluffs, truncated spurs,
meanders and open U shaped
valleys
Open and wide valleys with flat
floors
Slow flowing with pronounced
meanders.
Flood plains, meander loops, oxbow lakes, levees, deltas.
NOTE: the three stages shown in the diagram do not necessarily develop as shown as rivers flow through
a great diversity of physical features. Modern river studies use the idea of Dynamic equilibrium to
investigate rivers. This suggests that a river will erode and deposit materials at any stage along its course
if the right conditions are there for either of the processes.
FEATURES OF THE UPPER COURSE
1. Potholes: - if the river bed of a river is uneven, pebbles can get caught in a depression. The
flowing water moves the pebble round and round in the depression until it gets deeper to forms
a pothole.
2. Interlocking spurs – occurs when water flows around protrusions of the surrounding highlands
because water flows along the path of least resistance downhill. This causes the stream to
become winding as if meandering but these are called interlocking spurs.
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Rivers And River Processes
3. Waterfalls:- these are sharp breaks in the bed of a rive r channel which develop when a river
after flowing over a relatively hard rock meet a band of soft rock which is eroded quickly leading
to the lowering of the river bed.
Formation of a Waterfall




Water passes over hard rock and erodes the softer rock below
A plunge pool is formed and the river erodes backward as the water and rock swirl
around
Eventually the hard cap rock overhangs and collapses under gravity
This process continues over time and the waterfall moves backwards or recedes to
form a gorge
4. Rapids:- These occur in the same manner as water falls but in this case the band of resistant
rock deeps gently down stream leading to an increase in gradient and therefore the velocity of
the river. Rapids increase the turbulence of a river and hence its erosive power.
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5. Gorges:- It is a narrow deep valley which can develop at any stage in the river if the right
conditions exist. A gorge may be formed in the following ways;
a. A retreating waterfall – where a waterfall is formed as a result of a hard rock overlying a
soft rock, the hard rock may become unstable and eventually collapse. As this process is
repeated, the waterfall retreats upstream, leaving a deep steep sided gorge.
b. Where a river maintains its course across a highland that is being uplifted it will produce
a gorge at the uplifted section.
c. Where the roof of underground curves and streams collapse as is the case in limestone
areas.
6. River Capture/River Piracy: This develops when the upper course of a river which flows on a higher ground is diverted
by a powerfully eroding river on lower ground.
 The river with greater discharge and higher erosive power will capture the weaker river.
 The point at which the headwaters of the minor river changes direction is known as the
elbow of capture.
 Below this point a wind gap marks the former course of the beheaded stream. A wind gap is
a dry valley which was cut in the hills by the former river.
 The beheaded river is called a misfit stream as its discharge is far too low for the valley in
which it is flowing. An example of river piracy is the Pungwe River in Nyanga which captured
the headwaters of the Nyakupinga River.
FEATURES OF THE MIDDLE COURSE
1. Truncated/blunted Interlocking Spurs:- As the main process at this stage is lateral or sideways
erosion, the spurs are eroded backwards to form truncated spurs. This widens the rivers valley
which now has a U shape instead of the V of the upper course.
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2. Meanders:- these are loops or bends of a river channel. They are usually formed when a river
fails to erode resistant obstacles such that they start flowing in a winding manner.
Landforms associated with Meandering Rivers
As the water flows round the bend the flow is fastest on the outside of the bend where the river
is deeper. Here the river erodes the outside bank of the river and undercuts it to form a River
Cliff. On the inside of the bend the river is slow flowing and shallow. This causes deposition to
take place and a slip off slope or point bar is formed. In fact the meander is caused by
hydrocoidal flow and the water travels around the meander like a corkscrew
Meander Migration
This is where meanders move downstream. As the river continues to erode on the outside of the
bend and deposit on the inside meanders continually change their position. As you can see from
the diagram below, they help to form the flood plain and the ways they do this are:





The meanders have got wider due to erosion on the outside of the bends
The meanders have moved or migrated downstream
A line of river cliffs has formed along the edge of the valley floor
Deposition on the slip off slopes has built up alluvium (deposits) on the valley floor
As the meanders get wider so does the valley floor or flood plain
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FEATURES OF THE LOWER COURSE
As a river approaches its mouth i.e. a sea or lake, its velocity will decrease due to reduced gradient and
as a result, deposition will take place. Common depositional features include;
1. Flood Plains: - a flood plain is a flat gentle sloping plain of alluvium covering the floor of the
valley down which a river flows. When a river is in flood, it overflows its banks and covers the
floor of the valley on which it deposits sediment. The increase in the floor area covered by the
water increases friction which leads to deposition. Coarse materials are deposited first and later
the finer materials. Successive flooding causes the flood plain to increase in height and the thin
veneer of silt deposited by each flood improves the fertility of the soil.
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2. Levees: - Levees are basically caused by floods. In times of low flow any deposition takes place
on the river bed and this raises the height of the river bed.
In times of flood the water leaves the channel.
As it does so it loses energy and the courser
and heavier material is deposited near to the
river on the banks. Finer material is carried
further onto the flood plain. After many floods
the river builds a bank on either side of the
river and this can lead to catastrophes. As the
river has been raised above the flood plain and
the banks are often higher still, when the river
floods the water can no longer drain back into
the river channel and often has to form a new
one.
3. Deferred Tributaries: - Levees may
prevent a tributary from joining the main
channel thus causing it to flow parallel to the
main channel for a long distance in the flood plain. The tributary eventually joins the main
stream and they meet at what is called a deferred junction.
4. Ox Bow lakes/Cut off Lake: - this is an abandoned meander which has been detached
from the main channel due to erosion and deposition.
The Formation of Ox Bow Lakes
 In the lower course the meanders become even larger. Erosion on
the outside of bends and deposition on the slip off slopes
continue to take place.
 The erosion causes the meander neck to narrow until eventually
it breaks through. This often takes place when the river has more
energy as in times of flood.
 The river now follows a straight path. After the meander has been
cut through the river seals off the bend by deposition and an oxbow lake is formed. In time, especially in dry areas the ox-bow
lake may dry up to form a meander scar.
5. Braided Channel: - This is a result of a river getting choked by its load. This occurs when a river’s
energy suddenly falls due probably to the widening of the channel. The load is deposited leading
to the formation of flat islands which split the river into a series of diverging and converging
streams
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6. Deltas:- rivers are forced to deposit to immediately deposit their load as soon as they enter seas
or lakes because their flow is drastically reduced. The largest particles are deposited first and a
delta is formed. As the river continues to choke itself, distributaries wander off from the main
stream into the open water. The distributaries build their own levees so that they end up
flowing in raised channels.
Condition necessary for the formation of deltas
There are three main conditions required for the formation of deltas;
1. The river must be carrying a lot of physical load. This means there must be active erosion at
some points along the river channel.
2. The rate of alluvium deposition must be greater or faster than the rate at which the same
materials are removed by waves, ocean currents and tides.
3. The river mouth must be shallow so that the alluvium does not immediately sink.
Types of deltas
There are four basic types of deltas
1. Arcuate Delta: - these are fan shaped and the delta is formed outside the sea. They are
prevented from wondering out to sea by strong currents. They provide fertile land used for
intensive cultivation.
2. Bird’s Foot delta: - these are formed when offshore currents are very weak at the delta is
formed within the sea. The distributaries wander out to sea and may continue to push out to
sea for many kilometers e.g. Mississippi delta in New Orleans.
3. Estuarine Delta: - these occur when off shore currents are very strong so that deltas can only
develop in river estuaries. They assume the shape of a braided river channel though they occur
at the mouth.
4. Cuspate Delta: - are like a pointed tooth and occur in areas with moderate currents which allow
some deposition to take place.
RIVER REJUVINATION
This refers to increased vertical erosion in a river channel caused by a negative change to its base level.
The base level of a river is the lowest level to which a river can erode vertically. It is usually determined
by the sea level or an inland lake. A river obviously can not erode below sea level otherwise the sea
would flow backwards.
Causes of Rejuvenation/causes of changes in the base level of a river
1. Crustal uplift (folding and faulting) caused by plate tectonic movements which cause the sea to
fall in relation to the land. This will increase the gradient of the river leading to increased
erosion.
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2. A permanent increase in rainfall which permanently increases the discharge of a river channel
and therefore permanently improves its ability to erode vertically.
3. A permanent increase in Glaciation which reduces the amount of polar ice which melts into the
sea and therefore leads to a fall in sea level.
4. Localized volcanic activity which uplifts the land. This rejuvenates the channel.
Features Produces by river rejuvenation
1. River Terraces: - a rejuvenated river will cut deeper into its valley floor creating a new flood
plain at a lower level. What remain of the former flood plain appear as steps which hang above
the current flood plain. These steps occur on both sides of the river channel and are known as
river terraces.
2. Knick Point; - this is a point on the river bed where the rejuvenated river leaves the old flood
plain entering into the new plain. The knick point is usually marked by a waterfall or by rapids. It
may also retreat upstream leaving behind it a gauge.
3. Incised meanders:- if a meandering river is rejuvenated, it cuts deeper into the valley floor
giving rise to incised meanders.
Importance of Rivers/Benefits to men
1. They provide water supplies for domestic industrial and agricultural purposes. Dams have been
built across rivers to provide a continuous supply of water. E.g. The Zambezi River
2. Transport: - where rivers flow through out the year and have no waterfalls or rapids, they
provide important inland waterways especially in thickly forested areas in tropical rainforests.
3. Power: - water is used to generate hydro electric power.
4. Rivers also act as international or regional boundaries between countries and regions.
5. They provide fertile soils in their flood plain which supports intensive agriculture e.g. the Nile
River
6. Rivers are a source of commercial and subsistence fishing and therefore create employment and
provide protein rich food.
7. Rivers produce beautiful scenery and awesome features such as waterfalls and gorges and this
promotes tourism.
8. They are a source of minerals such as alluvial Gold in some parts of Zimbabwe and South Africa.
9. Rivers produce river sand and gravel which are important building materials
Problems faced from rivers
1. Rivers may lead to the formation of Marshy areas which make the land unsuitable for local
agriculture.
2. They may promote the spread of water borne diseases such as bilharzias, cholera etc
3. They create communication barriers where bridges are difficult to build or when in flood.
4. Flooding can be a major problem leading to loss of life and destruction of property
Causes of Fooding
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1. Natural causes
a. High rainfall
b. Melting of polar ice – Glaciation
c. Volcanoes and earthquakes which displace water in rivers
d. Rock impermeability which reduces infiltration and encourages runoff.
2. Human causes
a. Urbanization which covers land in tarmac and reduces infiltration
b. Deforestation and overgrazing
c. Poor farming methods such as stream bank cultivation which leads to siltation of rivers.
Reducing the effects of Flooding
1.
2.
3.
4.
5.
Planting trees and grasses which will reduce runoff and encourage infiltration
Using good farming practices such as contour ploughing and rotational grazing
Strengthening levees along rivers to keep water within the channels by building artificial levees
Building large dams which will collect most of the water which could have gone down stream.
Establishing and funding emergency response initiatives which will assist in times of floods to
reduce the loss of life
6. Harvesting the water collected in city drains rather than draining it to streams and rivers.
7. Spillways - These are overflow channels which allow rivers to flood areas of unused land or
areas which do not cause much damage
8. Flood Warning Systems -This enables people time to remove possessions and evacuate
areas.
9. Sandbagging - This attempts to flood proof homes and buildings as a last resort.
10. Insurance - This spreads the cost of the flood damage.
11. Flood Plain Zoning - This tries to organize the flood defenses in such a way that land that is
near the river and often floods is not built on. This could be used for farming. The areas that
rarely get flooded therefore would be used for houses, transport and industry
Rivers - Glossary
Key Words
Definitions
Alluvium
Material deposited by a river (sand-silt & mud)
Attrition
A process of erosion whereby the load rubs against itself
Channel
What a river flows in
Confluence
Where two rivers meet
Corrasion
Where the load of a river is used to erode the banks and bed
Corrosion
A process of erosion caused by solution ie salt and limestone
Delta
Discharge
Distributaries
Drainage Basin
An area of deposition found at the mouth of the river - triangular in shape
The amount of water flowing in a river per second
Small channels found on Deltas as the main river splits up
The area of land drained by a single river
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Rivers And River Processes
Estuary
Flood Hydrograph
Gorge
Hydraulic power
Hydrological Cycle
A drowned river valley
A graph showing how a river responds to different amounts of rainfall
A narrow steep sided valley caused by a waterfall
A process of river erosion caused by the force of the water
The cycle of water between the air land and sea
Lag time
The time between the peak rainfall and peak discharge
Meander
A bend in the river - usually in the middle or lower course
Mouth
Ox bow lake
Saltation
Where a river enters the sea or a lake
A lake formed when a meander is sealed off by deposition
Particles bouncing down a river bed
Source
The starting point of a river
Traction
Boulders rolling along the river bed
Tributaries
V-shaped valley
Velocity
Watershed
Small rivers or streams which flow into a larger river
Shape of a river in the upper course with steep sides and narrow base
The speed of a river's flow
High ground that surrounds a drainage basin
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