Channel types

Question 1

When does braiding occur and what factors form them?

Answer 1

-occurs when the channel is divided by islands or bars
-steep channel gradient, large proportion of coarse material, easily erodible bank material, highly variable discharge

Question 2

What are the causes of meanders?

Answer 2

-friction between channel bed and bank causes turbulence, stream flow unstable and this produces bars along the channel
-sand bars in the channel may cause meandering
-helicoidal flow causes the thalweg to move from side to side within the channel

Question 3

How are meanders formed?

Answer 3

1. in a straight channel, alternating areas of fast flow and slow flow will develop and the faster areas will have more erosive power and create deeper areas (pools, the slower areas will be shallower riffles
2. the thalweg will be deflected in low flow by the riffles onto the bank and start to create erosion on the bank, these riffles will be 6x channel width apart
3. helicoidal flow will lead to deposition inside of the bend creating a slip-off slope with erosion creating a river cliff on the outside

Question 4

How are waterfall created?

Answer 4

-occur where there is a sudden change in gradient, undercutting occurs creating a waterfall
-tend to form where a line of resistant strata (rock) overlay a softer rock, e.g. resistant limestones at Niagara Falls
-undercutting at the base of a waterfall creates a overhang, which will ultimately collapse thus a waterfall made migrate upstream leaving a gorge - Niagra Gorge is 11km long due to the retreat of the Niagra Falls