Erosion occurs when river is in flood. It can then carry huge amounts of material in suspension as well as being able to move the largest of boulders lying on its bed. Erosion can deepen and widen a valley. Deepens- vertical erosion
Characteristics:
V Shaped valley
Interlocking spurs
Waterfalls
A river in flood transports material, which cuts downwards quickly deepening the bed of the river. After periods of heavy rain, soil on the valley sides slowly moves downhill under gravity. Valley forms a V shape
Hard resistant surface rock is left unsupported as the river erodes the underlying softer rock more quickly. The resistant rock will collapse. The river, widening and deepening the plunge pool at the foot of the waterfall, will swirl around this material. Over a period of time as more rock collapses the waterfall will slowly retreat leaving a steep sided gorge.
Meanders and oxbow lakes formation:
River gets nearer to mouth – channel increases to hold more water, which the river receives from tributaries. The river gets bigger it carries more material (Small cause big broken up). Material will be carried in suspension and will erode the riverbanks by corrasion.
Meanders- large bends due to river flow on flat land
Lateral erosion – when river goes around a bend and the outside of the ped erodes. 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.
Ex: Mississippi:
Carries loads of material in suspension in times of flood. When it falls speed is low. Large amounts of slit fall out of suspension onto the bed of the river. In time the bed of the river builds up so that at times when river leves are High again, the river is more likely to overflow its banks.
Levée→ surrounding of a river
Prevention → artificial levées. Cities are protected.
Big rivers → transport big amounts of fine material in mouth.
falls into calm sea → speed will reduce and material will be deposited and form a delta. Deposition blocks main channel sot hey has to divide in smaller channels (distributaries)
Hydrological cycle:
Saturation: when soil is moisturized
Water table: level at which saturation occurs in ground/soil
Infiltration: transfer of water to soil from surface
Percolation: transfer of water to underlying rocks
Groundwater: water stored in rocks following percolation
Precipitation & run off:
Rainwater melted snow →
lost through evepotranspiration
Storage in lakes/soil/underground
Run-off
Precipitation – évapotranspiration + Storage)
Run off of a river will e less than precipitation.
Discharge → amount of water in a river channel at a given time.
Approach segment: discharge of river before rain
Rising limb: rapid increase in Rainwater reaching the river
Lag time: difference between time of heaviest rainfall maximum level of river
Falling limb: Rainwater is still reaching river but in down amounts
High flood risk – short lag time+ steep rising limb.
→ Precipitation reaches channel quickly due to surface run off. River has no time to transport excess H20
Low flood risk – long lag time + gentle rising limb
→ Rainwater reaches the channel slowly and over long period, lets river time to transport excess water.
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.
by
katioucha9 (student)
