River Processes and Pressures

Contrasts along a river's course
1. Rivers transport water and sediment from the land to the sea. Main purpose of eroding land
2. Upper Course
  1. Steep-sided valleys/gradients
  2. Water travels quite slowly because of friction from a rough, stony river bed
  3. Zig-zag, interlocking spurs
  4. Frequent rapids and waterfalls
  5. Clear water with little sediment
  6. Waterfalls- retreat because over time erosion and weathering lead the waterfall face to move upstream as worn away
  7. 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
  8. 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
  9. 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
3. Middle Course
  1. Meanders- flatter floored valley, Gradient reduced.
    1. 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
  2. Channel becomes wider, volume is increased, tributaries join. Valleys open out with gentle slopes and travel is faster!
4. Lower Course
  1. Meanders developed- Wide floodplain and ox bow lakes
  2. Channel is deeper and wider, water full of sediment, Water travelling faster
  3. 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
1. Gradient- Increases downstream
2. Velocity and discharge(speed and volume)- Both increase downstream, volume rises due to the small tributaries joining the main river flow
3. 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
4. Sediment- Eroded by attrition so it gets smaller downstream, Total sediment load carried by the river increases downstream
River land forms and processes
1. Erosion
  1. Hydraulic action - the force of water striking the river bed and forcing air into the cracks of rocks, breaking it apart
  2. Abrasion- rocks dragged by the water across the bed and thrown against the rocks against the banks and will wear them away
  3. Attrition- the rocks themselves will be worn down and broken up
  4. Corrosion - water will dissolve rocks such as limestone
2. Transporting
  1. Traction- rocks and other particles are dragged along the river bed
  2. Suspension- small particles are kept in the water itself until it stops moving
  3. 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?
1. The amount of water in the channel exceeds the channel capacity which causes it to overflow.
2. Storm Hydrograph
  1. Rising limb- IF the river water begins to rise a few hours after the rainstorm begins.
  2. Peak flow- About 26 hours after the start of the storm
    1. Then the recession
  3. Depend on Physical factors
    1. Long lag times- before peak discharge
      1. Permeable rocks e.g. sandstone
      2. Gentle slopes
      3. Forests- deep soils
      4. Long period of light rain
      5. As it was previously dry, the ground can absorb the rain
    2. Short lags and peak rapidly
      1. Impermeable rocks- clay and granite
      2. Steep slopes
      3. Grassland and thin soil
      4. Short period of heavy rain
      5. Ground saturated due from previous rain
3. How people cause flooding
  1. Urbanisation- Increases the area of ground where water is going to reach the stream by ovreland flow because concrete/tarmac isn't permeable
  2. Drains- deliver water quicker than soil
  3. If water reaches the river quicker than the peak discharge is going to be higher
    1. River will flood more often
  4. 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
Key terms
1. Confluence- The point where two rivers meet
2. Tributaries- Smaller river that flows into a larger one
3. Source- Where a river begins
4. Mouth- Where the river meets a lake or the sea
5. Watershed- Dividing line between drainage basins
6. Infiltration - When the water goes into the soil

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River Processes and Pressures

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	Created by georgia.somerville over 10 years ago