Rivers are the major agents of land sculpture throughout most of the world.  A river is a mass or body of water that flows through a definite channel down slope.  Rivers and their valleys vary in size and length from one part of the earth to another.  A river will develop if there is adequate rainfall and if there is a slope that will enable the water to flow down hill.

A point where a river starts to flow is its source.  The source of a river may be a spring, a lake or a marsh.  But it is generally in a highland region where precipitation is heaviest and where there is a slope, down which the water can flow.  The point where the river ends is known as the mouth.  Most rivers end in the sea.  Some rivers end in swamps or lakes and this occur in areas of internal or inland drainage.

The highlands, which receive high rainfall, form the catchments areas of rivers.  A catchments area is when a river begins its course downstream.  The crust of the mountains is the divide or water shade which separates the headstreams of different river systems.  A river receives tributaries during its flow to the sea and eventually evolves a system, which occupies a basin.

A number of factors work together to determine the development of a river valley.  These factors include the following:  the climate, initial slope and the underlying rock structure.  The climate influences the rain-fall amount and distribution.  The availability of a slope enables the development of a river because it is very hard for a river to develop from flat landscape.  The rock structure determines the degree of resistances to erosion of the rocks.

Types of rivers.

 The main source of river water is rainfall.  Thus, rivers may be divided into three groups.  These are:  Ephemeral rivers, Seasonal (intermittent) rivers and perannial (permanent) rivers.

1.Ephemeral river.   These are rivers which appear only, during the rainy seasons.  These type of rivers exist in areas which experience very little rain.

2.Seasonal (intermittent) rivers.  These type of rivers flow only during the wet seasons.  Such rivers occur in areas experiencing seasonal rainfall.  They may continue flowing even after the rains have stopped.

3.Perennial (Permanent) rivers.  These rivers flow throughout the year.  The source of such rivers is in with heavy and well-distributed rainfall throughout the year.

East Africa has many rivers both big and small, all of which rise somewhere in the huge plateau and mountain areas.  The most important rivers in East Africa include:  River Nule, River Aswa, River Katonga, River Kafu, and so in Uganda, River Athi, River Tana etc in Kenya and River Rufigi, River Ruvuma, River Pangani, River Malagasi and so on in Tanzania.  See map below showing major rivers.

The Major Rivers of East Africa.

 

 

 

 

TERMS USED IN RIVER SYSTEM

1.     River channel, is a groove or furrow through which a river flows.

2.     River discharge, is the quantity of water passing down a stream.

3.     River Regime, this is the seasonal variation in the volume of water of rivers and is determined by:-

• Seasonal precipitation
• Presence of snowfields and glaciers at the source.
• Nature of rocks (permeability)
• Character of vegetation
• Rate of evaporation.

4.     River competence.  This is the rivers ability to erode and transport the load. This is determined by the volume of water, angle of slope and nature of the load itself.

5.     Load.  The materials a river carries constitutes  the load.

6.     Velocity. This is the speed at which the river is flowing and is determined by the angle of slope.

7.     Catchment area of rivers, is where a river begins its course down stream.

8.     Divide or watershed is the crest of the main.  An imaginary line which separates the drainage basins on either side of a mountain range

9.     Arms which separates the headstreams of different consequent rivers.

See  diagram below.

 

 

 

10.                        An interfluves.  This is a piece of land which separates two subsequent rivers.

 

 

11.                        A consequent river.  This is the main river from the source to the mouth.

12.                        Subsequent river.  This is the river that joins the main one (tributary).

13.                        In sequent river.  Is the river that joins a subsequent river.

14.                        River System.  This is the main river and its tributaries.  See diagram below.

 

 

 

 

 

 

15.                        Drainage basin is the area occupied and drained by a river system.  See diagram below.

 

Drainage basin.

 

 

 

 

16.                        Long Profile of a river.  This is the appearance of a river from source to mouth.  It has three stages:   Upper Course, Middle Course and Lower Course.

Diagram Long profile of a river:

17.                        Cross Profile of a river is the appearance of a river from bank to bank.

 

 

 

 

18.                        River confluence.

19.                        Headwaters.  These are various streams that flow from the rivers source.

20.                        Intermittent rivers.  These rivers dry up from time to time.

 

STAGES IN RIVER DEVELOPMENT

Most rivers can be divided along their course into three fairly distinct parts.  These include:

1.     The upper course (youthful or torrent stage).

2.     The middle course (valley or mature stage).

3.     The lower course (old age/flood plain/senile stage).

 

The Upper (Youthful or torrent) stage:

The source of a majority of rivers is a mountainous area.  In such areas, rivers are characterized by:-  

• Fast flowing and cut deep, narrow, steep-sided V-shaped valleys
• Interlocking spurs occurs so that a river follows a zigzag course
• Potholes are cut by rapid circular movement of pebbles.
• Waterfalls and rapids develop where hard rock
• The river has few tributaries
• River has little volume of water and therefore concentrates on vertical erosion.

 

Middle (Mature or valley) stage:

In this stage the river is characterized by the following:-

• The gradient becomes gentler
• The river erodes the sides through lateral erosion leading to a wider valley
• floor because the river has got a number of tributaries which ................?
• In this stage the river has an open V-shaped cross-section.
• Meandering begins in this stage.
• Cliffs and slip-off slopes develop on the meander banks.
• Interlocking spurs are planed off to become bluffs.

 

Lower (old, plain or simile)stage:

• In this stage, the river has a very large volume of water and a heavy load.
• Deposition is the main function in this stage.
• The gradient is very gentle
• _ meandering is more pronounced.
• Deposition of sediments interrupts river flows creating braided channels.

Several landforms are found in this stage.

These include:

1.     Meanders

2.     Ox-bow lakes

3.     River cliffs and slip-off slopes

4.     Flood plains and levers

5.     Deltas and distributaries

6.     Incised meanders

7.     Braided channels

 

THE WORK OF RIVERS AND THEIR RESULTANT FEATURES

As rivers flow, they perform the triple function of erosion, transportation and deposition.  The extent to which rivers can successfully perform these functions depends on a river's energy.  The rivers energy is the ability of the river to erode, transport and deposit.

The energy of a river increases with increase in its volume /discharge and velocity (speed of water).  The greater the volume, the greater the river's energy.  And a fast flowing river has more power to effect erosion and transportation.

l. RIVER EROSION

This is the cumulative effect of a number of processes through which a river degrades and widens its channel.  The erosion action of a river consists of four interacting processes.

Hydraulic action.  This is caused by the shear force of moving water which can sweep out loose material from the banks and by surging into cracks the banks helps to break up solid rock.

Corrosion or Abrasion.  This is when the river uses its load as a "grinding tool", for wearing away the riverbed and banks.  The rock fragments are hurled against the sides of the river and also rolled along the riverbed.

Corrosion (solution).  This is the chemical or solvent action of water on soluble rocks with which the river comes into contact.  For example, calcium carbonate in limestone is easily dissolved and removed in solution.

Attrition:  This is the wearing down of the  transported materials themselves when they roll and collide with one another.  The load reduces in size during attrition and becomes easier to transport.

Types of river erosion.

There are three types of river erosion.  These are:

Headword erosion.  This is when a river cuts back at its source thus increasing its length.  See diagrams below.

 

 

Vertical erosion.  This is when a river digs into its bed through hydraulic, solution and corrosion processes increasing in depth.  See diagram below:

 

 

 

 

 

 

 

Literal erosion.  This is when a river erodes its sides widening its channel.  See diagram below.

 

 

 

 

 

Resultant landforms:

A number of landforms develop as a result of river erosion.  The main landforms include the following:

V-shaped valleys.  This is the valley cut by vertical erosion of the river.  It is narrow with steep sides.

Such valley is common on most rivers in East Africa.

Pot holes.  Potholes are deep circular holes formed on rive beds.  They are formed by a fast flowing stream which produces whirl pools in originally shallow depressions on a riverbed.  The whirlpools scour the shallow depressions forming circular holes on the bed called potholes. See diagram below.

 

 

 

 

 

 

 

Interlocking spurs.

A river may take the easiest path down the slope and tend to go round rocks that are resistant.  This results in a winding course of a river and bends become more pronounced eventually causing projections called "spur" that interlock.

 

Water locking spurs occur on all rivers in the youthful stage. A waterfall is a sharp break in the course of a river where water drops through a vertical distance in one fall.  Waterfall can occur at any part of the river course, but they are most numerous in the mountain course where changes of gradient are more abrupt and frequent.  Waterfalls are formed in the following ways  and can be considered as the main ones:

 

(a) Unequal resistance of hard and soft rocks lying across the river's bed horizontally and vertically.  The outcrop of a bend of hard rock may cause a river to "jump" or "fall" down stream.

 

(b) Faulting may cause a fall if a river drops over the edge of a fault scarp (escarpment).  For example the Kabalega falls on river Nile in Uganda.

 

(c) When a river descents s sharp edge of a plateau

(d) Glaciations produces hanging valleys where tributary streams reach the main U-shaped valley below as water falls.

 

(e) Where rejuvenation of a river valley has formed a sharp knick point a waterfall may form.  See diagram, occurrence of a waterfall.

 

 

 

 

Example of waterfalls in East Africa, Kabalega Falls, Bujagali Falls and Karuma Falls (River Nile), Sipi Falls (River Sipi) Sezibwa Fall (River Sezibwa) and so on in Uganda.  Thika Falls (River Thika) in Kenya.

 

Rapids.  These are sections of rough and fast flowing water in a river channel where the bed is steep and rocky.  Rapids form where the gradient of the channel is steep but without a sudden break of slope as it happens with waterfalls.

 

A river Gorge.  This is a narrow, elongated, steep sided valley.  Gorges are formed where:

(a) A river flows along a line of weakness.

(b) A river maintains its course across a landscape which is slowly being uplifted.

(c ) A river cuts across a plateau composed of horizontal and alternate layers of hard and soft rocks.

(d) A waterfall retreats upstream.

 

Below is a diagram for a Gorge.

 

 

 

 

 

 

 

 

 

A good example of the Gorge in East Africa is the Great Ruaha Gorge in Tanzania.

 

Uses of the Upper course.

1.                 The narrow V-shaped valleys are used in the development of barrages for storing water for irrigation and dam construction.

2.                 The waterfall and rapids have been tapped to provide H.E.P, for example the Nalubale power project on river Nile and Mobuku power project on river Mobuku in Uganda. The Pangani Power station and river Pangani in Tanzania, and seven Forks power project on river Tana in Kenya.

3.                 Features of the youthful stage promote tourism because they are scenic   attractions.

4.                 Some of these areas have been reserved for National Park Development e.g. Murchson Falls National Park in Uganda.

 

Problems posed by the Upper course of the river.

• Hindrance to navigation because of the existence of waterfalls and rapids.
• Narrow for large water vessels to be used.

 

2. TRANSPORTATION BY RIVERS

The amount of the load a river carries mainly depends on volume and velocity of the river.  The load is carried in the following ways.

 

(a) Suspension.  This is when fine materials float in water, for example sand and silt.  Materials carried in this way are called suspension load.

 

(b) Salutation.  Larger particles are transported by bouncing them along the bed.  Materials carried in this way are called Salutation Load.

(c ) Traction.  This is when the big stones and boulders are rolled along the ed of the river, hence the term ed load or traction load.

(d) Solution. Soluble materials like calcium carbonate, sodium chloride are carried away in solution.

(e) Floating.  This is the movement of materials at the surface of the water.  Materials like wood, leaves and other vegetable matter are transported in this way.

 

3. RIVER DEPOSITION

When the competence of the river to transport decreases, the load is deposited.  This results from:

- Reduction in gradient

- Decrease in the rivers volume

- Loss of velocity

- Obstacles in the channel

- Widening of the bed

-Rate of evaporation and so on.

 

Features of deposition:

Different landforms are developed by this process of deposition.  These landforms include:

Meanders.  These are curved bends of a river channel.  Their formation is not clearly understood but it is through to e related to one or other of the following:-

 

(a) Presence of obstacles. River rarely follows a straight curve.  Any chance of irregularities such as obstacles of hard rock may divert the river and develop a winding course as seen below.

 

 

 

 

The river will erode more on the concave (outside) bank and deposit on the convex

It is also said that meanders could be initiated by the alternate deep and shallow sections that form on the bed of a straight channel and cause the stream to swing from side to side as it is deflected by shallow sections.

 

(b)River cliffs and slip-off slopes.

River cliffs.   These are features found on the concave bank of a meander.  They are produced by erosion on this bank.  Slip-off slopes are found on the convex bank of a meander.  They are produced by constant deposition on the convex bank.

 

 

 

 

 

These features begin to develop in the middle or mature or valley course by both erosion and deposition.

 

Flood plain.  This is a wide gently sloping alluvial plain bordering a river usually in its old stage and contained between the bluff lines.  Rivers in their lower course carry large quantities of sediments.  During annual or sporadic floods, these materials are spread over the low-lying adjacent areas.  A layer of sediment is thus deposited during each flood, gradually building up a flood plain.

 

Levees.  They are associated with flood plain formation.  Leaves are embankments built alongside the river channel.  They rise above the level of the adjacent flood plain.  They are formed through successive flooding and deposition of sediments.  Deposition is greatest nearest the riverbanks because as the water floods out of the main channel its spread is immediately checked buy friction with the river banks and the heavier sediments are dropped first.  See diagram below:

 

Section of flood plain with levees.

 

 

Levees encourage lake and swamp formation on the flood plain.  Levees can be seen on the Ngaila, Nyando and Yala rivers on the Kano plains in Western Kenya and river Malaba in Eastern Uganda.

 

Economic importance of flood plain to man.

-The alluvial deposits give rise to very fertile soils hence promotes agricultural activities.

-The general flatness is ideal for mechanization.

-The proximity to the river makes it easy for irrigation to be practiced.

 

Problems associated with land rise in flood plain.

-There is a danger of flooding the surrounding plain.

-Flood plains provide good conditions for pests since they have stagnant water.

 

Ox-low lakes.