River Investigation

Erosion is a process whereby material carried by the river wears away the bed and banks. It causes the river dimensions to become wider and deeper, or in the case of river material, smaller and rounder

Erosion occurs through four processes:

* Abrasion- A sand paper effect, where materials rub along the bed and banks of the river causing them to erode. This process is more effective during times of flood and is the major method by which the river erodes both vertically and horizontally.

* Corrosion/ solution- A chemical reaction that happens between rock and the water e.g. the concentration of carbonic acid and humic acid. This happens continuously and is independent of river discharge and velocity

* Attrition- This is the erosion of the bedload of itself. Boulders collide with other materials causing them to break and become smaller and rounder in appearance.

* Hydraulic Action- Due to the force of the river, water is forced into cracks along the river bed. The air in the cracks is compressed therefore pressure increases and in time the bank will collapse. This is the slowest and least effective erosion process.

Depositional Processes

Deposition is where the velocity of a river looses its energy and is no longer able to carry it's load Material is deposited, starting with the largest particals.

Deposition occurs when:

* Discharge is reduced following a period of low precipitation.

* Velocity is lessened on entering the sea or a lake

* shallower water occurs on the inside of a meander

* the load is suddenly increased (caused by an increase in farming or a landslide)

* the river over flows its banks so that the velocity outside the channel is reduced ; resulting in a floodplain.

Transportation Processes

This is where excess energy is used to transport material down the river. As the velocity and turbulence increases, so does the ability to entrain materials.

The load is transported by three main processes.

* Bedload- This is where larger particals, which cannot be picked up by the current, are transported along the river bed. they are inturn transported in two ways: Saltation, where pebbles, sand and gravel are temporarily lifted up by the current and bounced along the bed in a hopping motion and Traction where cobbles and boulders are rolled along the bed. The very large material may only be moved at times of flood.

* Suspended load- This is where very fine particles of clay, silt and sand are carried by the turbulence of the river. The higher the velocity the bigger the particle that can be entrained. The suspended load is what gives the river its brown, black colour.

* Dissolved or solution load- This is where soluble rock such as limestone is dissolved and removed in solution by the constant running river.

Explanation of factors affecting a river.

* Basin size, shape and relief- If a drainage basin size is small, round and steep sided, rainfall will reach the river more quickly decreasing lag time and increasing velocity. ????????????

* Intensity of precipitation- The heavier the rainfall, the more saturated the ground will become therefore, not only will river fill up more quickly due to the heavy rainfall, but water will also flow faster to the river as surface runoff.

* Temperature- Extremes of temperature can restrict infiltration and so increase surface runoff. If evapotranspiration rates are high then there will be less water available to flow to the main river.

* Land use- Urbanisation- Has an increased flood risk due to water being unable to infiltrate through the tarmac or concrete. Farming- Over cultivation of the land surronding a river may mean that tyhe soil is more easily eroded causing a build up of silt in the river and an increased risk of flooding. The loss of vegetation may also mean a higher flood rate as water will flow more easily through the soil. .

* Vegetation- Large amounts of vegetation inhabiting the river, its banks and bed effects its velocity and therefore the amount of erosion.

* Relief- A higher relief will mean that materials will flow faster down the river.

* Geology

How rivers work

Precipitation ( River Basin Characteristics ( River Channel Characteristics

-Relief, Shape, vegetation - Depth, width, velocity, relief,

land use, Geology and soils, efficiency

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River Flow

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Energy

Equation = 1/2 mv ²

( Erosion

River Processes ( Deposition

( Transportation

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River Load River Features

- Oxbow lakes

- Deltas

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