Action of Running Water – SS2 Geography Lesson Note (First Term)

Running water is one of the most important agents of denudation. Rivers erode, transport and deposit materials as they flow from their source to their mouth. The work a river does changes along its three stages: the upper, middle and lower course.

Behavioural objectives

By the end of the lesson, students should be able to:

• explain the terms associated with rivers, such as source, mouth, confluence, tributary and river regime;
• state the factors that affect the velocity of a river;
• describe the upper, middle and lower courses of a river and name the features found in each;
• describe the processes of river erosion and river transportation;
• identify the types of drainage pattern and state the importance of rivers.

What are the terms associated with rivers?

1. Source of a river: The source of a river is where the river starts or begins, usually around highlands.
2. Course of a river: This is the path or channel through which the river flows.
3. Mouth of a river: This is where the river ends or enters the sea, ocean or lake.
4. River basin or catchment area: This is all the area drained by a river and its tributaries.
5. Watershed or water divide: This is the highland area which separates two or more rivers or two river basins. Rivers take their sources from the watershed.
6. River regime: This refers to the seasonal changes in the volume of water in a river in a year. A single regime has one period of high volume and one period of low volume. A double regime has two distinct periods of high volume in a year. Knowledge of a river regime helps people to control floods, store water for irrigation and human consumption, and plan H.E.P. production.
7. Confluence of a river: This is the meeting point of two rivers.

8. Tributaries: These are smaller rivers or streams that join a larger river.
9. Distributaries: These are channels formed by the division of a river as it flows into the sea. They are usually found in the delta region of a river.
10. River energy: This refers to the velocity of a river. The ability of a river to erode and transport the eroded materials depends very much on its velocity.

What factors affect the velocity of a river?

1. The volume of water released.
2. Slope of the river valley.
3. Shape of the river valley.
4. Amount and size of materials.

What are the stages of a river?

The entire length, valley or course of a river is divided into three main stages:

1. The upper course or mountain course (youthful stage).
2. The middle course or valley course (mature stage).
3. The lower course or plain course (old stage).

Upper course of a river

Characteristics of the upper course

1. It marks the beginning or source of a river.
2. It is found around highland areas.
3. It has steep sides.
4. The river flows swiftly down the steep slope.
5. The dominant work of the river is vertical corrosion or erosion.

Processes of river erosion

The load or materials carried by a river are the main agents of erosion, but the erosive work of a river consists of four processes. These are:

1. Hydraulic action: Fast-flowing water forces itself under pressure into cracks and joints within the valley and enlarges the cracks.
2. Corrosion: This is the wearing away of the sides and floor of the river with the aid of sand, pebbles, silt and boulders which are being transported. These materials eventually widen and deepen the river valley.
3. Attrition: This is the wearing down of the load as the materials collide with one another and with the floor and sides of the valley. Large boulders are broken down into small pieces like pebbles.
4. Solution: This is the chemical action of water on the materials it comes in contact with while flowing. For example, rock salt is dissolved and carried away in solution.

Features of the upper course

1. V-shaped valley
2. Gorge
3. River capture
4. Rapids and cataracts
5. Waterfall

Middle course of a river

Characteristics of the middle course

1. Lateral erosion is dominant over vertical erosion, resulting in the widening of the river valley.
2. There is an increase in the volume of water due to the addition of more water from tributaries.
3. There is an increase in the load of the river.
4. The work of the river is mainly transportation with little deposition.

Processes of river transportation

The load of a river is carried along the course of the river through four main processes. These are:

1. Solution
2. Suspension
3. Saltation
4. Traction

Features of the middle course

1. Wide V-shaped valley
2. Meander
3. River cliff and slip-off slopes
4. Interlocking spur

Lower course of a river

Characteristics of the lower course

1. The main work of the river is the deposition of materials.
2. There is active lateral erosion.
3. There is a lowering of the gradient of the valley floor.
4. There is a drastic reduction in the speed of the river.

Features of the lower course

1. Flood plain
2. Levees
3. Ox-bow lake
4. Braided river
5. Delta

What are the types of drainage pattern?

A river system, which includes the main river and its tributaries, may develop certain patterns within its basin. The types of drainage pattern include:

1. Dendritic drainage pattern: The tributaries, called the subsequent rivers, join the main river, called the consequent river, at an oblique angle. The pattern is tree-like, i.e. like the branches of a tree, and develops on rocks of the same type (homogeneous rocks).
2. Trellised drainage pattern: This is formed on alternate layers of hard and soft rocks. The tributaries follow the pattern of the rock structure and join the main river at an angle, forming a rectangular shape.
3. Radial drainage pattern: The streams or tributaries flow outward and down a hill, giving a pattern like the shape of a bicycle wheel.
4. Centripetal drainage pattern: Many rivers or streams flow from different directions into a lake.
5. Annular drainage pattern: The main stream or river almost forms a concentric ring around a highland, i.e. it almost flows around or encircles a hill.

Why are rivers important?

1. Rivers are a medium of transportation.
2. They generate hydroelectric power (H.E.P.) where rapids and waterfalls exist.
3. They provide water for irrigation.
4. They provide water for domestic and industrial purposes.
5. Floodplains formed by rivers provide fertile soil for agricultural activities.
6. Rivers provide food, e.g. fish, prawns and crabs.
7. They employ many people, e.g. fishermen and canoe builders.
8. Some rivers act as political boundaries between states, regions and nations.
9. Some rivers are centres of tourist attraction and may generate foreign exchange.

Evaluation

1. Define the source, mouth and confluence of a river.
2. State four factors that affect the velocity of a river.
3. Name the four processes of river erosion and explain any two of them.
4. List the features found in the lower course of a river.
5. State five ways in which rivers are important to people.

Assignment

1. With the aid of a diagram, describe river piracy.
2. With the aid of a diagram, describe the oxbow lake.