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. 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. 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.