How is a waterfall formed? Waterfalls may at first start off as rapids. The river flows over the hard rock while the soft rock beneath gets eroded. The erosion of the soft rock occurs more quickly than the hard rock, and this results in the hard rock being elevated above the stream bed which sits below. The erosion process of the rocks varies with the strength and density of the rock and the pace of the flowing river.Through a process known as abrasion, which is the erosion of rock surfaces due to friction between the rocks and moving particles carried by the river, a drop starts to develop. The vertical drop forms gradually as more and more of the soft rock gets eroded and carried downstream. Hydraulic action along with abrasion constantly keeps working to erode the softer rock and eventually a plunge pool is formed beneath the river. Gradually, the hard rock also starts getting eroded and carried downstream, which increases the size of the plunge pool.
Eventually, the river flow becomes steep enough and forms the familiar formation of a waterfall. The hard rock also breaks loose after gradual erosion and falls into the plunge pool. Here, it gets swirled around until a steep sided valley known as a gorge is formed. As waterfalls continue to flow over long periods, they begin to retreat. This results in a horizontal pit which is parallel to the waterfall. Over a period of many years, the pit becomes larger and the waterfall is replaced by a stretch of river bed, which is inclined. Movements of the Earth's crust, such as earthquakes and volcanoes, can cause a change in the land area, which also results in the formation of waterfalls. There are many different types of waterfalls and we will look at some of them briefly and how they are formed.