1. River Systems You will learn: Where do rivers begin?
How are river systems formed?
What are the parts of a river system?
What is a drainage basin?

2. The Course of a River
A river is a natural stream of water flowing in a channel
The source is the place at which a river begins
It may be melting snow from the top of a mountain or a lake with a stream flowing out from it
Due to gravity, the river flows downhill from its source
The path that a river follows is called its course
The part of the river near the source is called the upper course

3. The middle part of the river is called the middle course
Near the end of the river is the lower course
The place where the river flows into the sea or ocean is called the river mouth

4. The River System
Streams or other rivers may join the course of a river
They are called tributaries
The point at which a tributary joins the main river is called the confluence
The river and its tributaries form the river system

5. The Drainage Basin
Rivers drain the surrounding area by collecting surface run-off and directing it to seas or oceans
The area that is drained by the river and its tributaries is called the drainage basin
The boundary of a drainage basin is marked by surrounding highlands and is called the watershed

6. The Drainage Basin (cont’d)
The watershed separates one drainage basin from another and is also known as a divide

7. Action of Rivers
A river and the sediments it carries can erode the river bed and its banks
Over time, a channel is cut into the land
The channel eventually deepens to form a valley

8. Valleys One of the results of the action of rivers
The shape of a valley depends on how the river cuts into the land
In the upper course, the river is flowing down very steep slopes
Enough energy to cause great erosion
Cuts deep into the land and a V-shaped valley forms
A steep-sided valley with river flowing at the bottom is called a canyon

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In the middle course, the river flows across less steep land
Cuts less deeply into the land
Erodes the land sideways and downwards
Starts to deposit sediments
Sides of valley becomes wider
©MPH Education (S) Pte Ltd 2000
Slide 27.4

10. In the lower course, the land is almost flat
Bends in the river are very pronounced
More sediments deposited
Erosion of bends continues
Deposition of sediments along its channel results in a wide, flat-floored valley

11. Rapids and Waterfalls
River flows over alternating bands of hard and soft rocks
Soft rocks erode faster
Hard rocks jut out
A series of small and short falls develop
These falls are known as rapids

12. A waterfall is a vertical flow of fast-moving water from a great height

13. Rapids and Waterfalls (cont’d)
A waterfall usually forms when a river flows over hard and soft rocks
The soft rock is eroded more quickly
A ledge is formed over which water falls
The force of the water is so great that it cuts a depression or plunge pool at the bottom

14. Meanders Curves and bends in the lower course of the river
Formed when the river erodes sideways and deposits sediments on the opposite banks of the curves

16. Deltas
A river deposits its sediments when it flows into the sea or a lake
The deposits build up and block any further movement of the river
A river has to follow other routes called distributaries
Distributaries branch out from a river

17. Deltas (cont’d)
A delta is a flat, low-lying area where layers of sediments have been deposited over time
For a delta to form
The river must be carrying a lot of sediments
Currents along the coast must not be too strong to carry away the sediments faster than they can be deposited