Waterfalls are sheer vertical falls in the channel of a river. Examples of well known waterfalls are the Niagra Falls in USA/Canada and Powerscourt Waterall in Wicklow, Ireland. Waterfalls are found in the upper course of a river. They can occur further downstream during rejuvenation of a river system. Typically waterfalls occur where a river flows across a band of hard rock onto a band of softer rock. An example of hard rock is granite. The softer rock could be sandstone or limestone.
The waterfall is created because of differential erosion, that is, different rates of erosion. The softer rock is eroded more quickly than the harder rock. As the river flows downstream the softer rock is eroded more quickly as a result of the processes of abrasion and hydraulic action. The river carries a bed load of stones and pebbles. As the river flows, the bed load strikes the bed and banks of the river causing the channel to erode. This process is called abrasion. Hydraulic action is erosion caused by the sheer force of flowing water. Water rushes into cavities or small cracks in the river channel and forces soil and rock particles away from the river channel. In a waterfall, erosion is concentrated downwards into the bed of the channel. This aspect of the process is called vertical erosion and causes the river channel to deepen.
As the river passes the hard rock a knick point develops on the soft rock. Here the river water can become ‘broken’ by the development of rapids. This effect produces ‘white water’ because the water is turbulent here. Over a long period of time the soft rock will erode producing a vertical drop in the river channel. This is the waterfall. The energy of the falling water will continue to erode the bed of the river so that eventually a plunge pool will develop at the base of the waterfall. The plunge pool develops initially because the force of the falling water creates a slightly deeper pool in the bed of the river. As the rivers’ bed load falls into the pool it swirls about frantically and scours the base of the pool causing it to deepen.
Over a longer period of time, perhaps hundreds of years, the plunge pool will grow larger and assist the river in eroding backwards. This backward erosion is called headward erosion. Eventually the band of hard rock above the plunge pool will be undermined. Without the support of the rock below, the band of hard rock will collapse into the river. Over a still longer period of time, the river will erode back through its bed and produce a gorge. A gorge is a valley with high vertical walls at the bottom of which lies the river which eroded it.