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Erosion is the process that wears away the river bed and banks. Erosion also breaks up the rocks that are carried by the river.

There are four types of erosion:

• Hydraulic action - This is the sheer power of the water as it smashes against the river banks. Air becomes trapped in the cracks in the rock of the river bank and bed, and causes the rock to break apart.
• Abrasion - When pebbles grind along the river bank and bed in a sand-papering effect.
• Attrition - When rocks that the river is carrying knock against each other. They break apart to become smaller and more rounded.
• Solution - When the water dissolves certain types of rocks, eg limestone.

Weathering and mass movement

As the river flows, it erodes the land creating a valley with steep sides called a v-shaped valley. If the river meets more resistant rock it will flow around the rock. This produces interlocking spurs. The river erodes the valley bottom, but the valley sides are broken down by weathering.

There are three types of weathering:

• Mechanical – physical changes causes rocks to break down, for example, a change in temperature from day to night causes rock to expand and contract and break up (eg onion skin weathering). Or, changes in temperature cause water to freeze and expand, melt and contract leading to freeze-thaw weathering.
• Chemical – rainwater is slightly acidic and over time this wears away the rock.
• Biological - occurs when plants and animals wear away rocks, for example, roots growing in cracks in the rock or animals such as rabbits burrowing into rock.

The broken rock fragments (as a result of weathering) move down the slope through mass movements. These can be rapid, such as landslides or slow as with soil creep:

• Landslides are occasional, rapid movements of a mass of earth or rock sliding along a steep slope. They can occur after periods of heavy rain, when the water saturates overlying rock, making it heavy and liable to slide.
• Soil creep is a very slow movement, occurring on very gentle slopes because of the way soil particles repeatedly expand and contract in wet and dry periods. When wet, soil particles increase in size and weight, and expand at right angles. When the soil dries out, it contracts vertically. As a result, the soil slowly moves down slope.