River Processes and Pressures - created from Mind Map

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Contrasts along a river's course

Rivers transport water and sediment from the land to the sea. Main purpose of eroding land

Upper Course

Middle Course

Lower Course

Steep-sided valleys/gradients

Water travels quite slowly because of friction from a rough, stony river bed

Zig-zag, interlocking spurs

Frequent rapids and waterfalls

Clear water with little sediment

Waterfalls- retreat because over time erosion and weathering lead the waterfall face to move upstream as worn away

1. Undercutting by hydraulic action and abrasion. 2. Gravity leads to the rocks falling 3. Boulders are moved around to carve out a plunge pool through abrasion. 4. Waterfall retreats upstream over time 5. Creates a steep-sided valley or even a gorge

Steep valley sides are weathered by processes such as frost weathering/ Mass movement such as rockfall and sliding takes this material to the stream where it forms part of a load

Streams cut down vertically. Bed is strewn with rocks and debris which are moved only after storms. Material wears down bed and wandering course of the river creates interlocking spurs

Meanders- flatter floored valley, Gradient reduced.

Channel becomes wider, volume is increased, tributaries join. Valleys open out with gentle slopes and travel is faster!

Meanders- formed when the faster flowing water on the outside of the bend either erodes whilst on the inside of the bend/deposition (river loosing energy and dropping everything) meanders become more obvious/oxbow lakes form when the neck of a meander becomes so narrow that it cuts through it from the flood leaving an old meander there. Deposition soon blocks up the bend creating a lake that slowly fills up

Meanders developed- Wide floodplain and ox bow lakes

Channel is deeper and wider, water full of sediment, Water travelling faster

Floodplains develop as rivers meander from side to side pushing back the valley sides to create bluffs. When meandering, they spread deposits across the valley floor and widen it as they occasionally erode against the bluff lines. When rivers floor, new deposits spread out over this material with the coarse material settling first by the channel. Creates raised ridges known as levées.

Changes in channel shape and characteristics

Gradient- Increases downstream

Velocity and discharge(speed and volume)- Both increase downstream, volume rises due to the small tributaries joining the main river flow

Chanel characteristics- Channels become wider and deeper downstream. Channel bed gets smoother and is less efficient due to the friction becoming less between the water and the channel sides

Sediment- Eroded by attrition so it gets smaller downstream, Total sediment load carried by the river increases downstream

River land forms and processes

Erosion

Transporting

Hydraulic action - the force of water striking the river bed and forcing air into the cracks of rocks, breaking it apart

Abrasion- rocks dragged by the water across the bed and thrown against the rocks against the banks and will wear them away

Attrition- the rocks themselves will be worn down and broken up

Corrosion - water will dissolve rocks such as limestone

Traction- rocks and other particles are dragged along the river bed

Suspension- small particles are kept in the water itself until it stops moving

Solution- Material is dissolved in water e.g. salts

Gorges have near vertical slides because there is little weathering or mass movement, the river cuts downwards rapidly; weathering and mass movement operate fast, rivers erode slowly

Why do rivers flood?

The amount of water in the channel exceeds the channel capacity which causes it to overflow.

Storm Hydrograph

How people cause flooding

Rising limb- IF the river water begins to rise a few hours after the rainstorm begins.

Peak flow- About 26 hours after the start of the storm

Depend on Physical factors

Then the recession

Long lag times- before peak discharge

Short lags and peak rapidly

Permeable rocks e.g. sandstone

Gentle slopes

Forests- deep soils

Long period of light rain

As it was previously dry, the ground can absorb the rain

Impermeable rocks- clay and granite

Steep slopes

Grassland and thin soil

Short period of heavy rain

Ground saturated due from previous rain

Urbanisation- Increases the area of ground where water is going to reach the stream by ovreland flow because concrete/tarmac isn't permeable

Drains- deliver water quicker than soil

If water reaches the river quicker than the peak discharge is going to be higher

Deforestation- Increase the amount of rain that reaches the ground. Reduces evaportation and transpiration, more surface run off into the river which increases flood risk

River will flood more often

Key terms

Confluence- The point where two rivers meet

Tributaries- Smaller river that flows into a larger one

Source- Where a river begins

Mouth- Where the river meets a lake or the sea

Watershed- Dividing line between drainage basins

Infiltration - When the water goes into the soil

River Processes and Pressures