How a river grows

A river, in some ways behaves like a human being. Just as a human being grows upwards after being born so does a river. On taking its rise, the river begins to grow headwards by a process called headward erosion. It achieves this through four processes spring sapping, undercutting, headward down cutting and river capture. I will explain these one after the other:

Spring sapping is the process whereby the spring uses its hydraulic force as it issues out from the rocks under pressure, to erode away the rocks surrounding its source. At this happens, the spring source recedes uphill, and so the river grows uphill.

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Undercutting is the local erosion of a layer of weak rocks underneath a resistant rock ledge. The resistant rock ledge overhangs and with time collapse and falls by gravity. this causes the river valley, and therefore its source, to recede uphill. This occurs in areas with alternating bands of bed rock, made up of varying levels of resistance. Some types of rocks (shale, for example) wear away easily than others (such as sandstone or limestone).

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Headward down-cutting is caused by flood water flowing in a channel down the slope. As it tumbles over a small waterfall at the head of the valley, it cuts down the valley at this head, thereby making the channel grow uphill. Gullies grow this way.

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Capturing also makes a river grow, when it cuts off and diverts the headwaters of another river into its own , thereby adding to its own length. This can happen as a result of tectonic earthmovements, natural damming, erosion and glacier retreat.

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All these processes cause the river valley and therefore the river itself to elongate or grow headwards in the major process of headward erosion.

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