River system is to some extent established. River capture or river piracy takes place, in which case, one river reaches the course of another one and if the course of the second river is diverted because of the greater gradient of the earlier river, this phenomenon will be known as river-piracy.
The point where the course of the second river is diverted is known as ‘elbow of capture’. The captured river is called the ‘misfit-river’ and the abandoned part of the channel, through which no water flows is the ‘wind-gap’.
In this stage, there is conspicuous formation of V-shaped valleys.
In this stage there is maximum erosion through lateral-cutting. The landmass is fully dissected; ridges and valleys develop strikingly.
As a result of heavy erosion and deposition this stage is comprised of a large number of landforms such as hog-backs, castes, plateau tables, meanders, ox-bow lakes, terraces, alluvial fans etc.
Down-cutting stops at the base level of erosion, which is the mean sea level produced inland. Frequently the river is over-flooded and builds up flood-plains on both sides. The initial irregular surface has become practically flat at this stage.
The river is mostly engaged in depositing and does little of erosion and transportation. The topography is characterised by a set of distributional features which comprise delta, distributaries etc.
Features and typical-landforms associated with an ideal fluvial cycle in a humid region:
When the width of the valley is greater than the width of the river, the valley is called a strath.
During the process of regrading, there is a more or less marked change of slope at the point of intersection of newly graded profile with the older one, and it is known as the knick-point.
Due to pronounced erosion by running water the areas are intricately traversed by gullies, which mostly develop on argillaceous rocks.
It is a steep slope resulted from differential weathering of rocks.
In a region of sub-horizontal beds, a gentle slope is developed along the gentle dips of strata, such a landscape is known as cuesta.
An isolated table-land area with steep sides.
With continued erosion of the sides a mesa is reduced to a smaller flat-topped hill, known as ‘butte’.
It is a cuesta, in which the dip slope and scrap slope are both approximately 45°.
In this case distributaries or branches develop in large number in a region of flatness. They are commonly formed where the amount of load is excessive and the stream is incapable of transporting all of it. The coarser fractions of the load tend to form islands in the centre of the stream, which breaks up paths around them. Thus a braided river is formed.
The peneplains are formed in the old stages of rivers and are the plain lands produced by the river.
Sometimes some mounds or small hillocks of hard rock persists on the peneplains and are knows as monadnocks.
(xii) Natural levee:
On the flood-plains, long depositional ridges extending parallel to the river are found, which are known as Natural-levees.