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Revision notes on rivers, erosion and floods

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Introduction

Geography TEST revision Main features: River basin is an area of lad drained by a river. And tributaries. Higher land around river? watershed. Rain falling in High areas will fall into a Stream developing into a river. Beginning of river ? Source Joint in a river ? Tributary Joint with river ? confluence End of river (sea/lake entering) ? mouth Shape of land by rivers: Material can end up transported into the channel. River can transport it downstream. Causes erosion. More erosion = more material available for river. EROSION: Attrition: material is moved along the bed of a river, collides with other material and breaks up into smaller pieces Corrasion : fine material rubs against the river bank. The bank is worn away by a sand papering action called abrasion and collapses. Corrosion: acids in the water dissolve some rocks forming the banks and bed of a river. Hydraulic action) sheer force of water hitting the banks of the river. ...read more.

Middle

The outside of the bend is the deposition. Erosion narrows the neck of the land within the meander. Eventually during a flood it twill cut through the neck. The fastest current will now tend to be in the centre of the river and so deposition is likely to occur in gentler water next to the banks. Deposition will block off the old meander to leave an oxbow lake. It will dry up after a while. If a river is slow it will bend a lot. Mississippi+ Amazon has oxbow lakes Mouth approach Flood plain Flat area of land over which a river meanders As the speed at which the river flows across the flood plain is less than in the main channel, some of the fine material transported in suspension by the river will be deposited. When a river flows a thin layer of silt or alluvium is spread over the flood plain. River floods ? coarsest material first. Forms Small embankments along a river. ...read more.

Conclusion

Causes of floods: Physical (type/amount precipitation ? type of soil underlying rock) Human (land use of the river basin / human activity) Main cause ? heavy rainfall. Ground becomes saturated and infiltration will be replaced by surface run-off. (follow thunderstorms-) Flashflood : when ground is too hard for rain to infiltrate and surface run off causes river levels to rise Snowfalls ? water is held in Storage. Flood risk is big if large raise in temp. If the rise is accompanied by a period of heavy rain, ground remains frozen prevent infiltration. Soils & rocks Permeable + impermeable ? sandy (infiltrates water) + clay (no) If impermeable flood risk is higher Land use: No vegetation ? High flood risk Trees intercept rainfall delaying the time and reducing the amount of water reaching the river. Human activity: Disforestation + urban growth. Impermeable tarmac + concrete replace fields and woods and makes infiltration reduced. Drains and gutters re constructed to remove surface water. Decreases time taken by Rainwater to reach the river but increases flash flood risks. Small streams travel along culverts/pipe. May not be large enough to cope with Rainwater fall. ...read more.

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