Geography: Rivers

Erosion

Hydraulic Action: Movement of Sediment by frictional drag of moving water. Flow lifts loose sediment as velocity increases. Velocity high = material removed from banks leading to undercutting and collapse.

Abrasion: Rubbing of bed and banks by sediment carried along by river. Where depressions exist, turbulent flow can cause pebbles to swirl (eddying) creating potholes.

Attrition: Reduction in size of sediment particles as they collide with each other, the bed and banks. Further downstream = sediment becomes rounded and smaller.

Corrosion: Occurs as rocks dissolve into the water and are carried away. Most common in areas with carbonate rocks like Limestone.

River can erode vertically and horizontally. Vertical erosion is characteristic of faster-flowing rivers, as there is sufficient energy to downcut, producing rapid lowering of channel floor and generate steep-sided valleys. where there is a sizeable floodplain, the river may meander, meaning lateral erosion will dominate.

Transportation

Energy that remains after frictional drag becomes available to transport material, usually the sediment that has been loosened by erosion or washed into the valley sides.

Traction: Larger materials are moved by rolling or sliding along the channel floor

Saltation: Materials ranging from pebbles to sand grains are temporally lifted and bounced along the floor in a hopping motion.

Suspension: (Main form of transportation) Comprises of fine muds and clays are held in the flow of the water, with the lightest material nearer to the surface.

Solution: Where sediment is dissolved into the water due to weak acids, like carbonic acid from precipitation, acting on more soluble rocks, for example chalk.

Hjulstrom Curve

This illustrates the relationship between velocity and competence. It shows the velocities at which sediment will be normally be eroded, transported and deposited. The mean or critical erosion velocity shows the velocity required to to pick up and transport sediments of varying calibre. The mean fall or settling velocity curve indicates the velocity at which each calibre of sediment is deposited as the energy level is too low.

• Fine Particles ~ higher velocities before eroded. Velocity needed to keep in suspension is low, so they can stay in suspension even when water is still.
• Large Particles ~ High velocities before eroded. High velocity to be able to transport them, slight drop in velocity will mean that they will be deposited.

Deposition

River deposits material when it is no longer competent. any reduction in river velocity will reduce competence and material will begin to be deposited, starting with the coarsest sediment as this needs a lot of energy to remain in suspension. This means upper course is filled with large boulders whereas in lower course, finer sediments dominate.

Deposition frequently occurs when:

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