When a stream or river plunges over a wall of rock called a cliff or a recipice, we have a waterfall. If the waterfall is of great size, it is called a cataract. Where the rock wall is steeply slanted rather than vertical, the rushing water is called a cascade.

Sometimes in a cascade, the water descends in a whole series of steep slopes.

Niagara Falls is an example of how an overhanging rock ledge can create a waterfall. The upper layers of rock at Niagara are hard beds of dolomite. Below the dolomite is weak shale. The Niagara River plunges over the dolomite cliff into a great pool below, where the swirling water wears away the shale and thus undermines the dolomite above. From time to time, great masses of dolomite fall, keeping the cliff fresh and steep. In other waterfalls of this type, the hard rock may be sandstone, limestone, or lava.

Another type of waterfall is illustrated by Lower Yellowstone Falls. A huge mass of molten rock was squeezed up from below in ancient times. It hardened and later formed a wall in the path of the river’s course. In some cases, ancient glaciers cut deep into mountain valleys, leaving the sides as steep cliffs and precipices from which the waterfalls plunge down. In still other cases, high plateaus have been lifted by movements of the earth’s surface and the streams plunge over their edges.

The three most famous cataracts in the world are Niagara Falls, Victoria Falls in the Zambesi River in Africa, and Iguassu Falls between Argentina, Brazil, and Paraguay. Of these three, Niagara Falls has the greatest volume of water.

The world’s highest waterfall is Angel Falls in Venezuela, which plunges 1,005 metres down. This waterfall was first seen by Jimmy Angel from a plane in 1935, and was first visited in 1948.

Some waterfalls are very useful to man in providing power. The falls are used to generate the electricity man uses to run factories. About half the world’s potential water power is in Africa, but most of it has not yet been developed.