In the middle course of a river vertical erosion generally stops, because the river has no capacity to pick up and transport more material. In this part of its journey to the sea, a balance is achieved between its load and velocity.
The river, therefore, can only erode laterally, picking up material on one side and depositing it on the other. In dry season, when the volume of water shrinks and velocity decreases, sediment accumulates in the channel. It is picked up again during the next flood period.
The part of a river valley having a flood plain but still having definite valley walls, is called mature. The mature river’s course is zig-zag forming loops/meanders on the valley floor.
It starts eroding its bank on the outside of each meander loop and depositing material on the inside of each loop. The net result is that the size of the meanders is increased by this act of the river. So the floodplain becomes wider.
The land enclosed by meanders becomes flat and is covered by the alluvium during periods of falling velocity after the flood has subsided. The lateral erosion continues and the size of the meanders is increased. The land covered with alluvium is also increased.
As the water flows round a bend, the river tends to increase the curve, since current acts more strongly on the concave side or outside of the curve, so that maximum erosion takes place.
There is little erosion, and even some deposition on the inside of the bend. Thus, the original swing is transformed into fully developed meander with a river cliff overhanging the under-cut bank and a sloping spur, called a slip off slope projecting from the opposite side.
It is to be borne in mind that the size of the meanders and the width of the meander-belt are related to velocity and the bed-load of the river. At this stage the lateral erosion continues more vigorously widening the valley.
Weathering is an additional factor which continually assists in the valley widening. Spurs are also undercut and ultimately each spur is removed. As the river approaches the old age, it begins to create a broad and almost level valley bounded by low walls of the bluffs.
An important landform in the middle course of a stream is an ox-bow lake, which is crescent-shaped and develops in the flood-plain. Actually, this has been a part of the meander as described above. This meander has been cut through and abandoned by lateral erosion of the river banks at the meander’s reach.
A portion of the stream channel is cut off as the stream tends to flow in the shorter, steeper, and straighter path. Actually this cut off section of the meandering channel forms the ox-bow lake. The ox-bow lake is also referred to as a bayou, billabong or mortlake.
Flood-plain is a part of the river valley, adjacent to the channel over which a river flows when there is flood in the river. It is a zone of low relief and gentle gradient and may incorporate ox-bow lake, point bars, abandoned channels and scrolls.
These features indicate that the river channel has shifted its positions continuously during the present regimen of the stream. A flood- plain is composed of alluvium which generally covers the rock floor of the valley to different depths. The alluvium deposits were formed either within the channel itself or as over bank deposits in times of flood.