After streams have formed and have started to cut their valleys, many things may happen. One of these happenings is the river capture or stream piracy. River capture is of very common occurrence in such regions as are characterized by folded rocks.
River capture of youthful and mature rivers is a common feature of the regions of folded rocks. Besides, stream piracy is also common when streams meander widely over their flood-plains. This is due to lateral erosion and intersection of meanders.
How river capture occurs has been clearly illustrated. There are three consequent rivers flowing across a series of tilted rocks. Stream B, as shown in the figure, has much more discharge than streams A and C, therefore it cuts its valley deeper than the valleys of the other two rivers. It develops head-ward tributaries in more easily eroded rocks-shown by S (soft rocks).
These tributaries have lower bottoms than those of streams A and C. So the tributaries of the stream A cut through to streams A and C. Consequently, the bottoms of the valleys are lower than the valleys of streams A and C, so those streams are diverted to B and are said to be captured.
A stream whose upper waters have been captured is said to be beheaded. The valleys of A and C just below the point of capture, now being dry (having no water) are called wind gaps or dry gaps.
Below the wind gaps, there may be a new supply of water thus continuing the flow of water of streams A and C as shown. There are cases when below the points of capture the valleys will have no streams in them. At the point of diversion there is usually a marked bend, known as elbow of capture.
The river which has lost its headwaters will be much reduced in volume, and therefore too small for its existing valley, hence it is called a misfit stream. Such a captured river may be so much smaller that its source will now be some distance below the point of capture. In exceptional cases, as stated above, the valley remains dry.
River-capture or stream piracy obviously takes place readily in escarpland areas, where subsequent streams can cut back along the belts of less resistant rocks at right angles to the consequent streams, so capturing neighbouring head-waters, and producing a trellised drainage pattern of great complexity.
According to Prof. Woodridge, river-capture does not need special structural conditions to occur. As a matter of fact, river-capture is a normal incident in a struggle for existence between rivers.
There are many examples of river capture or stream piracy such as the beheading of the Black-water by the Wey River, capture of the upper Doubs from the Rhine, and several remarkable captures in the scarplands of the Paris Basin.
In reality, the development of contiguous river systems must lead to one river becoming more powerful than its neighbours; slowly it becomes the master stream of the area. It is accomplished by watershed- regression and diverting part of a nearby system into its basin.