River Capture or Stream Capture

In the case of river capture or stream capture, one stream actively attacks and erodes the drainage of another stream which results in the diversion of parts of its waters to pirating stream. The main reason for piracy is the capacity of one stream to preserve and extend its valley at a lower level than that of an adjacent stream.

This means that either by chance or structural advantage, one stream cuts into the drainage basin of another stream, progressively or abruptly cutting off or diverting new tributaries. This process is also termed capture, and the trunk stream that is deprived of its stream is said to be beheaded.

Stream piracy may occur due to abstraction, headward erosion, lateral planation and subterranean diversion (Thornbury, 1997).

Commonly stream capture is observed in subsequent drainage systems capturing the earlier consequent drainage.

Consider two streams flowing across a coastal plain with their valleys separated by interfluves if Stream A is shorter than Stream B but more powerful and aligned in such a way that the headward erosion of this stream will project in the direction of the valley of Stream B. Due to the headward erosion of Stream A, the drainage divide between the two valleys will reduce. Progressive erosion and extension of Stream A in the headward direction into Stream B will eventually take place.

Since Stream A is more powerful than Stream B, it captures the waters of Stream B. Stream A becomes the captor, and Stream B is the beheaded stream. The upper part of Stream B becomes the captured stream, and the abandoned channel of the abrupt bend of Stream B is the elbow of capture (McKnight, 1998).

Let us consider an initial consequence that is superposed on a folded belt. The subsequent valley expands along weak rock belts, progressively intersecting the previous consequent drainage. The initial consequent by this time may be a series of graded reaches alternating with gorges or water gaps across the resistant strata. After river capture, the former course is marked by wind gaps (Bloom, 1978).

Shenandoah River, a tributary of River Potomac, through headward erosion along the weak rock of Blue Ridge, captured waters of streams flowing through Snickers, Ashby and Manassas areas of the Appalachian mountain of United States. The Shenandoah River was flowing at a level lower than three other streams and was cutting its valley at a faster rate (Thornbury, 1997).

Another example can be cited of the Niger River in West Africa. The headwaters of Niger are near the Atlantic Ocean, but the river flows inwards rather than seawards. It flows for about 1600 kilometres in the northeast direction, then turns abruptly in the southeast and continues on its journey for another 1600 kilometres before reaching the Atlantic Ocean. It is believed that the upper part of what is now the Niger River was a separate river that did not change course but flowed northeast till it emptied its waters in the inland lake in Central Sahara. River, after being beheaded by ancestral Niger, produced the great elbow of capture and left behind a beheaded stream that, in due course of time, dried with the rise in aridity (McKnight, 1998).

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