Waterfalls 101

The following image illustrates the different forms of waterfalls you may encouter around the world. Different regions might refer to them by different names, but all types are represented here.

What Causes Waterfalls to Form?

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.

Are Waterfalls Permanent?

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.

Where are the Highest Waterfalls?

In 1935, an American aviator, Jimmy Angel, was prospecting for gold in southeastern Venezuela when he discovered treasure of a different sort. Flying his small plane up a narrow canyon, he came upon a plume of water plummeting off a sheer cliff. Its source proved to be a river that spills off a high plateau. Subsequently named Angel Falls, the cascade is the highest in the world. It drops a total of 3.212 feet.

Steep escarpments have produced other notable waterfalls, including Angel's closest rival; Tugela Falls in South Africa also plunges off a plateau, with a total drop of 3,110 feet. The third highest in the world, Yosemite Falls in California, tumbles from the mouth of a hanging valley for a total distance of 2,425 feet.

Which Falls are Most Powerful?

High waterfalls may be spectacular, but in terms of volume of flow, the most powerful are generally wide and relatively low. Among the titans is Khone Falls, on the Mekong River in Laos. It drops only 72 feet, yet has an average discharge of 400,000 cubic feet of water per second.

But perhaps the greatest of all "falls makers" is the Rio Parana in South America. One of its tributaries is the site of Iguassu Falls, pouring over a cliff more than two miles wide. Although its average discharge is only about 60,000 cubic feet per second, the figure increases to more than 400,000 in times of flood. Downstream on the Parana itself Guaira Falls - with 7 1/2 times the average discharge of Iguassu - was the mightiest cataract on earth until it was flooded out of existence by the construction of the Itaipu dam, completed in 1982.

How was Niagara Falls Formed?

Although far from the highest, Niagara Falls is among the most famous in the world. Located near the midpoint of the Niagara River, which flows from Lake Erie into Lake Ontario, the falls are twin cascades with a total drop of about 180 feet. On one side of an island in the river is the long, straight line of American Falls; on the other, the graceful crescent of Horseshoe Falls.

The falls came into being about 10,000 years ago, at the end of the Ice Age. Originally they fell over an escarpment capped by an extremely hard layer of dolomite. As plunging water wore away softer underlying layers of shale and sandstome, large sections of the cap rock collapsed. Over the years, continued undercutting of the dolomite has caused the falls to migrate about seven miles upstream to their present site. In time - but not for at least 25,000 years - the falls may retreat all the way to Lake Erie and disappear entirely.