Whether slender ribbons or seething walls of foam, all waterfalls owe their existence to an abrupt change in the level of a river's channel. Many of the world's greatest falls - in South America, Africa, and India, for example - spill off the margins of broad, elevated plateaus. Heavy rainfall assures an ample supply of water for rivers that course across the plateaus and have nowhere to go but down when they reach the edge. Or the cliffs may have been formed by crustal movements. After the earth's surface moved up or down on opposite sides of a fault, rivers that once flowed across flat land were forced to leap down escarpments. Steep mountain slopes, too, are laced with numerous waterfalls and rapids. Some of the most spectacular are found in mountains that have been carved by glaciers. Where small tributary glaciers joined a main valley glacier, the main valley was often deepened into a steep-walled trench. The tributary glaciers carved much shallower depressions that were left perched at the tops of cliffs as "hanging valleys." Rivers now pouring from the mouths of such valleys form some of the lovliest falls in the Alps. The fjords of Norway, too, are adorned with plumelike waterfalls that spill from hanging valleys.

Every waterfall is doomed to disappear. The process is a gradual one with many variations, but the life cycle of a waterfall generally follows either of two basic patterns. One involves the slow cutting down of a resistant ridge of rocks as a river seeks to smooth irregularities in its channel. Where this happens, a single waterfall may evolve into a series of smaller cascades. Further erosion then hones down the cascades into a stretch of turbulent white water, or rapids, that eventually blends in with the smooth flow of the rest of the river.

In other cases, the top layer of rock may be harder that those beneath it, forming an erosion-resistant "cap." Then most of the erosion takes place at the base of the falls, where falling water carves out a deep plunge pool, or basin, in the riverbed. At the same time, churning debris wears away the weaker rocks in the lower part of the cliff. In time the cap rock is undermined and beraks off, sometimes in masive chuncks, and leaves a new crest slightly upstream. As the process is repeated, the falls slowly migrate upstream, often leaving a series of plunge pools in the riverbed that mark the former locations of the falls.