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Rivers Notes

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RIVERS The drainage basin is the catchment area from which a river obtains its water. It has inputs and outputs that create the hydrological system. Inputs - solar energy and precipitation Outputs - evapotranspiration, runoff into the sea, water percolating into the ground Stores - glaciers, rivers, lakes and puddles. Vegetation through interception, and water held in permeable rocks Transfers - overland, through and base flow Evapotranspiration - the total amount of moisture that is removed by evaporation and transpiration from a vegetated land surface

Storm hydrograph Steep rising limb - when the intensity and duration of the storm is high, so there is increased precipitation and more river discharge - more antecedent rainfall Permeable rocks - produce a lower rising limb as water can pass through the rocks, and therefore water travels more slowly through the system Small drainage basin - produce a shorter lag time as it responds very quickly to a storm Steep sided basin - water reaches the channel quickly which produces a steeper rising limb and shorter lag time High temperature - increased rate of evapotranspiration and therefore river discharge is reduced More vegetation - increased interception of water therefore the peak discharge is much lower

How does a river transport its load?
Traction - large stones and boulders are rolled along the river bed by water moving downstream Saltation - small stones bounce or leap-frog along the channel bed, pushed by the flow of the river water Suspension - very small particles of sand and silt are carried along by the flow of the river. The material is picked up by the turbulence of the water Solution - dissolved minerals are transported within the mass of the moving water Capacity - measure of the amount of material that the river can carry, the total volume of the load Competence - the diameter of the largest particle that the river can carry for a given velocity

Deposition A river deposits its load when it is no longer competent to carry it. It usually occurs when:

Reduction in the gradient of the river

Discharge is reduced

Shallow water

Increase in the size of the load

The river floods its banks

Hjulstrom Curve

Powerful velocities are required to pick up smaller particles such as clay and silt because they stick together and lie on the bottom of the river bed, making it hard for the water to carry them away

Once picked up, particles can be moved by much lower velocities - however large material, such as pebbles and boulders still require a high velocity in order to be carried by the water. Therefore, these particles will be deposited very soon after they are picked up

Smallest particles, clay and silt, are deposited at very low velocities

Flocculation - fresh water river meets the salt water of the sea which causes chemical settling of the clays and silts, as a result this creates extensive areas of mudflats

River load Dissolved load - soluble materials carried as chemical ions in the water Large material, such as boulders, only form part of the river load after extreme events, such as floods, which have lead to a significant increase in the stream discharge. Following a period of heavy rainfall or a flash flood, the river has the competence to carry larger material because the velocity is much higher Lower course - more of a capacity to transport more material because it is wider and the velocity it much higher in the lower courses of the river Example - The Mississippi River has a drainage basin the size of one third of the USA - every year it transports 136 million tonnes of load in solution, 340 million tonnes in suspension and 40 million tonnes by saltation

Factors affecting variations in load Size of the drainage basin - larger drainage basins, particularly with many tributaries, has a greater potential to carry more load

Rock type - if the underlying geology of the drainage basin is a soluble rock (limestone), more material is transported as dissolved load Relief - in a drainage basin with high relief, where there is a big difference in altitude between the source and base level, the river has more potential to transport Precipitation - when there is low rates of precipitation, there is generally low loads found in the basin Human activity - if there is deforestation in the area, this reduces interception and therefore increases the amount of load in the basin. Many farmers use nitrates and phosphates for fertilisers which can enter the river basin, adding to its load

Valley profiles Long profile - illustrates the changes in the altitude of the course of the river from its source to its mouth. The long profile is generally smoothly concave, with the gradient being steeper in the upper course Rejuvenation - occurs when sea levels fall or land levels rise, when the river renews its capacity to erode in a vertical direction

Cross profile Valley cross-profile - the view of the valley from one side to another

In the upper course, the valley is very narrow and steep-sided and the river occupies the entire valley floor. It is a result of vertical erosion

In the middle course, the valley becomes wider with valley bluffs and a flat floodplain. It is a result of lateral erosion

In the lower course, the valley is very wide and with a flat floodplain and no distinct valley sides. There is a lack of erosion and so there is large scale deposition

Graded profile - over time, a river will display an even and progressive decrease in gradient down the valley - produces a smooth and concave shape that is adjusted to the discharge and load of the river

Changing channel characteristics Upper course

Channel tends to be very narrow and rough due to increased deposited sediment

Both banks are subject to erosion and therefore the channel will be very shallow

Shallowness of the channel means there is more contact between the bed and banks and the water, which creates friction and as a result, produces a large wetted perimeter

The friction in the channel means a loss of energy and a subsequent loss of velocity

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