The following landforms are the result of the erosional activities of rivers:

1. Gorge 4. Rapids

2. Waterfalls 5. V-shaped Valleys

3. Potholes

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1. Gorge:

Gorge is a form of V-shaped valley which is deep and narrow with precipitous rocky walls occupied by a river. It is more steep-sided and enclosed than a ravine. As stated earlier, in the upper course of a river corrasion is greater than deposition.

The river, therefore, deepens its channel and forms a valley. If the rocks into which the valley is cut are hard and resistant, the sides or valley walls will be steep and the valley will be narrow.

The steepness of the valley walls is also influenced by the amount of rain. It is partly on account of dryness of the climate that the Colorado Canyon (a form of gorge) is very narrow and its walls are nearly vertical.

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Gorges are also developed when a powerful river cuts down at the same rate as the surrounding land is uplifted. The gorges of the Indus River in Kashmir and of the Brahmaputra where it crosses from Tibet into Assam, offer the typical examples of world’s great gorges.

Another striking example of the erosive power of the upper course of a river is seen in the Hindukush Mts. where a tributary of the Oxus River has carved a deep gorge whose sides for about 2 km. are almost vertical.

2. Water falls:

Waterfalls represent a point in the long profile of a stream where the water des­cends vertically. In fact, they mark the knick-point of the most impressive kind. Remember that waterfalls and rapids are generally found in moun­tain and plateau regions.

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Waterfall may develop when a river flows over a cliff caused either by a fault scarp or by resistant rock encountered by the river while deepening its valley. Hanging valleys also water falls. Streams flowing through plateaus built up by layers of successive lava flows generally develop waterfalls.

The famous Victoria Falls on the Zambezi River in South Africa is a fine example of such a waterfall. There is a lot of variation in height, form and volume of water. Sometimes it so happens that the point at which a tributary joins the main river is much lower, in that condition a water fall will be formed.

Where a bed of more resistant rock, horizontal or gently inclined upstream, overlies softer rocks, undercutting of the softer beds leads to undermining and recession of the waterfall. When- ever a waterfall recedes there is a gorge below it, because head-ward erosion is more rapid than the lateral erosion of the sides of the valley.

For example, take the case of Niagara Falls, where a stratum of hard dolomite limestone overlies the softer shale’s and sandstones. The scouring action of water at the base of the fall undercuts into the soft rock, so that undermining causes pieces of hard rock to collapse, and the fall recedes upstream.

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Remember that waterfalls are phenomena of the early stages of valley development, being really features of youth, like the gorges and canyons. A fall that descends in a series of leaps is called a cascade, whereas a cataract is a series of stepped waterfall which is larger than cascade.

3. Potholes:

Pothole is a more or less circular hole in the rocky bed of a stream formed by the scouring and grinding effect of pebbles rotated in an eddy formed at the base of waterfall.

In rapids and at the base of waterfalls, potholes are formed in solid rock through the grinding action of sand and pebbles which lodge in slight initial depressions and are swirled around by the swift water.

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As the pebbles swirl round they strike against the rock gradually drilling a hole into the solid rock. Some of these holes have a diameter of several feet. Formation of Potholes plays active role in causing the waterfall to recede.

This is made possible by the swirling waters which excavate beneath the overlying resistant layer, thus undermining it. Although the fall recedes, it does not disappear, rather it gradually migrates upstream.

4. Rapids:

Rapids occur on the steep slopes of a stream and are often developed due to recession of waterfalls. If successive rock-layers of unequal resistance lie across the course of a stream, there is sudden increase in the gradient as a result of which the stream velocity becomes abnormally high. Such rapids are generally found in the upper course of the streams.

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5. V-shaped Valley:

V-shaped valleys are carved out on the land surface by fluvial erosion, and their cross-profile approximates to a V-shape. These valleys have evenly sloping sides. When the rate of deepening of the stream channel slows down, valley widening due to lateral erosion and other factors begins to take place.

Now, the valley cut by a river in its upper course begins to assume a shape of the letter V of the English alphabet. However, in less resistant rocks widening on a larger scale accompanies deepening from the very beginning.

However, in the lower course the valley becomes broad and its bluffs recede back, so the newly created cross-profile will not be V-shaped.