Middle River Landforms

Question 1

Riffles and Pools

Answer 1

Form in low flow conditions where energy, velocity and competence is low deposition of alternating bars of sediment occurs.

Water is forced to flow around these bars; here friction increases as the water is shallower, decreasing energy, velocity and competence and so point bars form. Deposition occurs on the inside banks creating a convex bank.

Riffles create a swing of flow which directs maximum velocity towards the edges of the banks on the outside bends; erodes and undercuts creating concave bank. This erodes the channel so it is deeper as the fastest flow has more erosive power. These are pools.

Question 2

Meanders

Answer 2

1. Found in the middle/lower stretches of a river when vertical erosion changes to lateral erosion as river reach base level.
2. Form where alternating bars of sediment are deposited in low discharge. Energy is lost and velocity decreases when water flows over this sediment due to friction leading to deposition. These are point bars.
3. Water flows faster around the outer bend of the meander where increased velocity erodes the banks due to high energy. Pools are formed on the outside bend from sediment removal from lateral erosion and high erosive power.
4. Spacing between pools and riffles creates uneven flow so swing of flow directs max velocity towards banks.
5. Turbulence and velocity increases around the pools so the water coils and twists. This is called the helicoidal flow which moves in a corkscrew like motion transporting sediment from the outer bend to the inside bend of the next curve.
6. Eventually river cliff is formed on the outside bend from sediment removal from lateral erosion.
7. Meander migrates due to thalweg eroding banks, increasing sinuosity of river and creating an additional bend. This process repeats.

Question 3

Oxbow Lakes

Answer 3

1. Current strongest on outside bend of meander so rapid erosion reduces the neck of the meander as the sinuosity increases.
2. The neck meets as river breaks through and flows around the meander and through where the outside bends met.
3. the current along the straight path cutting out the meander becomes dominant and the old meander is abandoned.
4. This creates an oxbow lake as former channel sealed off via deposition and the old path res leaving the area near the river cliff full of stagnant water.
5. This eventually dries up leaving a meander scar, this is because he water is still and so silts up.