Fluvial Landforms – Middle course of river. River Channel: wider and deeper Volume of water greater than that in upper course. Gradient is less than upstream portion of the river course. Fluvial Landforms – Middle course of river. River Valley : More open V shape and wider valley floor.
Meanders, river cliffs and slip-off slopes
How meanders begin to form is uncertain.
They appear to have their origins in times of flood and in relatively straight sections where pools and riffles develop.
Pools: areas of deeper water.
Riffles: areas of shallow water.
Spacing between pools and rivers usually very regular: 5x – 6x of river width
There is less friction in pools.
Thus, river flows faster and has more erosive power.
Increased friction in the riffles slows water down and more deposition takes place.
Continuous erosion in the pools and deposition in the river accentuates the slight bends of a river.
There exists a series secondary flow of water in a river, other than the main flow.
One of these is called the helicoidal flow– cockscrew movement of water.
Difference in velocity across the channel causes unequal pressure distribution.
Current from the concave bank descends downwards, undercutting and eroding the river side.
Eroded material carried along the bed and up the convex bank where it is deposited.
This current represents a cockscrew movement of water because it continues downstream and repeats the series of rotations.
As erosion continues to undercut the concave bank, eroded materials slump down into the river, forming a steep river cliff.
Continuous deposition on the convex bank makes it shallow and deposition takes place.
These deposits on the convex bank build up into a gently sloping slip-off slope.
The channel has an asymmetrical cross-section.
Continued erosion on the concave banks and deposition on the convex banks cause the meanders to migrate laterally.