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River Processes and Pressures










River Systems

The river has three different "courses", the Upper Course, Middle Course and Lower Course, each with their own different characteristics.
  Upper Course Middle Course Lower Course
Gradient  steep less steep  shallow gradient 
Discharge small large very large
Depth shallow deeper deep
Channel Shape narrow with steep sides flat with steep sides flat floor, gentle sloping sides 
Velocity quite fast fast  very fast 
Valley Shape steep sides  flat with steep sides  flat with gentle sloping sides 
Features waterfalls, interlocking spurs.  meanders, floodplains.  meanders, floodplain, levees, ox-bow lakes 

Processes shaping rivers

Weathering and mass movement are the main processes that shape river valleys.

Weathering

  • Freeze-thaw - In the cold winter months, when rain water enters a crack in a rock and freezes, the freezing process exerts pressure on the rock eventually causing it to break into smaller pieces.
  • Biological  - Tree roots can penetrate cracks in rocks and split them apart
  • Chemical - All rainwater is slightly acidic and this can dissolve some rocks, especially limestone and chalk.

Mass movement

  • Soil creep - Particles of soil slowly move down the sides of valleys due to gravity
  • Slumping - The river erodes the valley sides making them steeper and increasing the downward movement of material. Heavy rainfall can trigger this movement.

Upper course landforms

In the upper course, there are a few unique landforms, mainly caused by erosion, which you are not going to find in the other courses of the river.

Thornton Force waterfall, North Yorkshire
"Thornton Force" by Val Vannet, is licensed under CC BY-SA 2.0 /
Modified from original
                      1. Mudstone - Where rivers flow over bands of tocks with different resistance, the softer rocks will be eroded more quickly. The River Twiss flows over limestone rocks and meets a band of mudstone. The mudstone wears away more quickly, creating a step which gradually becomes deeper.
                      2. Overhang - Eventually the overhanging resistant rock will collapse, making the waterfall steeper. If this happens repeatedly, the position of the waterfall moves upstream, known as retreat.
                      3. Undercutting - The water flows down over the resistant rock but when it reaches less resistant rock underneath, it erodes it, cutting back into the rock and creating the overhang above.
                      4. Plunge pool - At the foot of the waterfall is where you'll find a plunge pool. It's an area of deeper water that has been created by the rapid erosion of the mudstone as the waters plunge down. The higher the waterfall, the greater the speed of erosion due to the higher speed the water is travelling at.

Lower course landforms

Unlike the upper course, the middle course has both erosion and deposition acting upon it, but by the lower course, the main process acting on it is deposition.

Meanders

By Maksim (Own work)
via Wikimedia Commons

Oliver Kurmis [CC-BY-SA-2.0-de], via Wikimedia Commons
In a meander, the river swings from side to side and the force of it doing so directs the fastest current and greatest force of 
water against the outside of the meander, forming a steep bank called a river cliff. On the inside edge of the meander, the current is slower and deposition of sand takes place, creating a gently sloping bank called a slip-off slope.

Ox-bow lake

  • Narrow neck of land is eroded
  • Narrow neck of land is cut through and water takes quickest route. An ox-bow lake is formed.
  • Deposition seals off old meander and ox-bow lake begins to dry up.


Causes and impact of flooding
There are several different reasons as to why a river may flood, some of these are:
  • Heavy rainfall
  • Impermeable rocks
  • Snow melt
  • Deforestation
  • Urbanisation
  • Climate Change
BBC Article - What have we done to make the flooding worse?

York Floods, November 2000 - Impacts

  • Homes and businesses flooded
  • People were evacuated and the army were called in
  • Road and rail links were cut off
  • Huge cost. Cost over £1 million to deal with and millions in insurance pay outs.

Managing river floods

Hard engineering methods

  • Embankment or levee
    • Stops flood water from spreading
    • Expensive
    • May burst or may go over
  • Deepening and/or straightening of the river
    • Water flows away more quickly
    • Delays flooding
    • May be a scar on the landscape - unattractive
  • Dam built upstream
    • Able to regulate and control the flow of water
    • Expensive
    • Can burst
  • Relief channel
    • Takes overflow water
    • Expensive
    • Disruptive and complex to build

Soft engineering methods

  • Washlands
    • Cheap
    • Water can go somewhere without causing much damage
    • Can be used for special habitats
  • Plant trees (afforestation)
    • The trees reduce the amount of water reaching the river and the time it takes to do so
    • Provides wildlife habitats
    • Land cannot be used for anything else
  • Planning
    • Building restricted to areas with a low risk of any flooding
    • May cause planning problems elsewhere
  • Flood warning system
    • Prepare people
    • Sometimes it is not possible to give people enough warning