CORE UNIT: PATTERNS AND PROCESSES IN THE PHYSICAL ENVIRONMENT
because they are faster and larger than normal and have immense energy. They can cut deeply into the river channel (vertical erosion) and carry huge quantities of sediment. The Mississippi River carries one million tonnes of sediment into the sea each day.
River gradient and channel width and depth
The speed of a river is affected by the steepness of the slope (gradient) down which it flows, by the depth of the water and by the width of the river channel. It is also affected by the roughness of the river bed.Youthful riverswith large boulders in their channel flow more slowly than older rivers with smoother channel beds. Deep rivers flowing down sloping land are very powerful with lots of energy.
Shallow rivers flowing over flatter ground are less powerful. This is because of the drag (friction) between the river with its bed and banks.Wide and deep rivers flow faster than shallower and narrower river channels.
PLAN VIEW Bank SIDE VIEW Fastest
Slowest
Slowest flow
Bank
Fig. 13 The effect of friction on a river’s flow: the speed of a river changes as you cross from one bank to another.
Fastest flow
Friction with the river channel slows the flow
Fig. 14 The River Dee in Llangollen, Wales, in flood Processes of river erosion
The processes of river erosion are very important. Erosion can occur at the river banks (lateral erosion) and at the riverbed (vertical erosion), causing awidening and deepening of the river channel. River erosion processes are (a) hydraulic action, (b) abrasion, (c) solution and (d) attrition.