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

A waterfall with annotated features
  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

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

Causes and impacts of flooding

There are several different reasons as to why a river may flood, some of these are:

BBC Article - What have we done to make the flooding worse?

York Floods, November 2010 - Impacts

Managing river floods

Hard engineering methods

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