Lesson 26 A River System and itsDrainage Basin You will learn: where rivers begin how river systems are formed what the

Lesson 26 A River System and itsDrainage Basin You will learn: where rivers begin how river systems are formed what the parts of a river system are what a drainage basin is Earth Our Home 1

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

The Course of a River (cont’d) 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

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

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

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

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

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

Valleys (cont’d) 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 Slide 27.4 ©MPH Education (S) Pte Ltd 2000

Valleys (cont’d) 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

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

Rapids and Waterfalls (cont’d) A waterfall is a vertical flow of fast-moving water from a great height

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

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

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

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

Lesson 26 A River System and itsDrainage Basin You will learn: where rivers begin how river systems are formed what the