1 Unit 2 River processes produce distinctive landforms in their valley

2 The drainage basin What is a watershed? Flash animation of how rain falling on to the landscape will drain into a river.

3 River channel: the area in which a river flows. Tributaries: smaller rivers which join the main river[ increase its discharge] Confluence: point at which rivers meet. Drainage basin: land drained by a river system. Watershed: boundary of the drainage basin, usually made up of highland. Discharge: amount of water passing a specific point at a given time and is measured in cubic metres per second [cumecs]. Depends on the river’s velocity and volume. The volume is the amount of water in the river The velocity is the speed of the river.

4 Processes of erosion

WEATHERING Weathering is the breaking or the decomposition of rocks in situ. 5

THE MAIN TYPES OF WEATHERING. [3] Physical weathering: the rock is broken apart without any change to the minerals which form it. Chemical weathering: decomposition of rock [change in composition] by biochemical processes. Biological weathering: breakdown and/or decomposition of rock due to the chemical and/or physical agents of an organism [plants or animals] 6

Processes Physical weathering: water expands when changed into ice  expansion puts pressure on the rock around it  fragments of rock break off. Chemical weathering: weak acids in rainwater react with calcium carbonate in rocks  rocks break up or dissolve. Biological weathering: plants grow in cracks  roots break up rocks  may cause cliff collapse. 7

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Impact of physical weathering 9

10 BIOLOGICAL WEATHERING

Lichens and mosses grow on bare rocks and help the chemical breakdown of the surface of the rock.

12 CHEMICAL WEATHERING LIMESTONE PAVEMENT

Acid rain

14 B) Mass movement Definition: When material moves down a slope due to the pull of gravity. 1- Soil creep: very slow movement, very little impact on landform formation.

SOIL CREEP: Processes 15 Gravity pulls the water that is contained in the soil down a slope The soil moves downhill with the water. The slope appears rippled. These ripples are known as terracettes.

Slumping 16

17 SLUMPING

2- Slumping [rotational slipping] 18 Involves a large area of land moving down a slope. Very common on clay cliffs. Clay dries and cracks in summer  When it rains, water runs into cracks  absorbs by clay which becomes saturated  clay slips down.

How and why do the characteristics of a river valley and channel change downward 19

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22 Reasons for these changes: Discharge increase as the river flows downhill to the sea because tributaries join the main river  increased river energy. Friction decreases from upper course to lower course  higher velocity, increased power of erosion. Video on impact of channel roughness Video on impact of channel roughness Upper course: only vertical erosion as the river is small, most of the water is in contact with bed  95% of energy used to overcome friction  small velocity and small volume of water= lowest discharge.  V-shaped valley Middle course: lower gradient  lateral erosion[ higher discharge + transported material increase erosion; lower friction as discharge is higher and less water is in contact with the river channel] Lower course: still lateral erosion. But low gradient, deposition most important process  valley wider and flatter.