Combined Effects of Erosion & Deposition

• Pools and Riffles• Meanders and Ox-bow lakes• River Terraces

Pools and Riffles

Pools: are areas of deep water and greater erosion. (Energy build up due to less friction)

Riffles : are areas of shallow water caused by deposition of coarse sediment

Sinuosity

Is the curving nature of a river’s course.

Thalweg

The position of the rivers fastest flowing current.

(Least friction, greater energy, greatest erosion)

The meandering ThalwegThalweg – the line of fastest flow in a river

Pool

Riffle

Meander

Meander – Bends in course of a river channel

How occur?1. Begin when a river

approaches its middle course & gradient channel is less steep.

2. It results from helicoidal flow with faster current spirals downstream in corkscrew fashion.

3. Movement result in erosion on outside bend of meander to form river cliff and deposit on inside bend called slip off slope.

Point bar

River cliffSlip-off slope

Thalweg

Material is deposited on the inside of a meander bend as a point bar.

At point bars

particles are usually graded in

size (sorted) with the largest

particles found on

the upstream

side.

As erosion occurs on the outside of the meander bend, the whole meander migrates downstream.

Material forming

the point bar

becomes a

contributory factor

in floodplai

n formation

.

As sinuosity increases,

during flood events the flow may cut of the meander

bend at the neck, thus shortening its course.

Temporary straightening of the channel means the main flow is in mid channel, meaning deposition now occurs at the river banks and the old curve is therefore abandoned to leave an ox-bow lake.

River / Fluvial Terraces

• Benches on the valley side

• Formed when a river cuts into a former floodplain Following vertical erosion caused by rejuvenation

• Can be formed in bedrock

• Result from AGGRADATION f0llowed by INCISION of a river valley

• T = terraces• C = confluence