Stages in the Development of a River Valley

The river cuts downward to form a ‘V’shaped valley. The river starts to meander

River uses its meanders to cut from side to side eating into the valley Floodplain starts to form 1 2

Floodplain River bluffs Alluvial deposits River cliff

1 2

Why do V-shaped valleys occur?

Waterfalls!

Waterfall formation

Braiding Braided streams have a number of channel segments i.e. where the river has split and has small islands (eyots) mid channel. Can occur in the upper and middle course. The gradient is steeper than where you find meanders. There is large bedload and periods of fast flowing water (i.e. storm conditions).

Meanders and Ox bow lakes Objectives Understand the process taking place on a river bend Learn the features of a river bend(meander) cross section.

Meander formation Stage one Rapids( riffles) or shallows in the river deflect the fastest flow of water to one side of the river.

Stage 1 The fast flowing water begins to erode one side of the river. On the other side the slower flowing water begins to deposit silt and sediment (load)

Stage 2 Fastest flow erodes the outside bend, undercutting the bank creating a steep river cliff. Deposition on the inside of the bend creates a shallow slope (slip off slope)

Stage 3 Lateral erosion continues i.e. the bend cuts across the flood plain. The meander loop grows in size. Eventually a narrow neck of land may develop.

Stage 4 In times of flood the river may cut across the narrow neck of land. This may eventually lead to the formation of an ox bow lake.

Oxbow lakes

Oxbow lakes new course of the river Meander neck becomes smaller When the river floods it breaks through the thin meander neck and the river takes the easier, straight course. This leaves the meander loop ‘cut off’ as an oxbow lake. Over time, the oxbow lake will become colonised by vegetation. new course of the river Meander neck becomes smaller oxbow lake

Before…..

Waterfall (Knick point) retreats cutting Isostatic uplift causes land to rise. Waterfall (Knick point) retreats cutting a lower valley River terrace Original sea level