Sample Answer for Rivers

The processes of river erosion involved in the formation of a waterfall include hydraulic action, abrasion, head-ward erosion, differential erosion, and eddying. An example of a waterfall is the Torc Waterfall in County Kerry.

A waterfall, or an interruption in the river profile, often forms along the upper course of the river, where it takes a significant vertical drop. Waterfalls form over bands of soft limestone and hard granite rock. Due to differential erosion, soft rock erodes rapidly through hydraulic action and abrasion. Hard, resistant rocks are left elevated as a result and stand out above the riverbed. A series of rapids often form at this point.

As the river gradient increases, the water begins to flow over a lip of hard rock. An over-deepened plunge pool forms at the base of the waterfall, as the water pounds against the softer rock on the riverbed. Eddying, the swirling of pebbles in the hollow further increases its depth. The splashing of the falling water starts to erode the back of the waterfall, creating an overhang which will eventually collapse, causing the waterfall to retreat upstream through the process of head-ward erosion. This leaves a steep-sided river valley called a gorge.

Waterfalls may also form after glaciation; when a river returns to a hanging valley, it falls into the U-shaped valley, joining the main river channel and creates a waterfall. Waterfall may also occur in sporadic places due to rejuvenation, where a drop in sea level causes the river to return to the youthful stage. It erodes a deeper channel, and a waterfall is formed at the point where the renewed erosion occurs. Lastly, if tectonic uplift of plates occurs along a river course, a waterfall may form at that point.