Kyle Dunn, Jr., age 12, of white Plains, N.Y., for his question:
HOW ARE WATERFALLS MADE?
A waterfall is the sudden sheer descent of a stream or river over a steep drop in its bed, sometimes in a free fail. Waterfalls develop in several ways. Different rates of erosion where a resistant layer of rock in a stream bed overlies softer layers is the principal manner.
Subsequent erosion of the softer rock by the failing water then undermines and periodically breaks off portions of the harder cap rock. Some of the largest fails in the world, Niagara Falls in North America and Victoria Falls in Africa, for example, originated this way.
Waterfalls in mountainous regions generally develop where a glacier has deepened a major trunk valley, leaving less eroded branch valleys hanging. The tributary streams in these valleys discharge into the main river by falls or cascades.
Hanging valleys also develop where a main river deepens its channel more rapidly than its tributaries do. Such falls are among the highest in the world. Bridal Veil, Ribbon and Upper and Lower Yosemite Fails in Yosemite National Park, Multnomah Falls on a minor tributary of the Columbia River in Oregon and Gavarnie in France, for instance, ail descend from hanging valleys.
Other waterfalls originate where i fault uplifts a mountain range or part of a range, creating a fault scarp over which streams drop steeply. Continued undercutting and erosion of the edge and the rock bed above that falls move many waterfalls upstream. These diminish in size, dwindle to rapids and then disappear.
The term "cataract," usually designating a series of rapids in a large river, is often applied to waterfalls of large volume. A waterfall of any height that has a small volume of water or is one of a series of falls is called a cascade.
The term "cascade" is also applied to a waterfall if, while plunging, it maintains contact with the stream bed. The highest waterfalls frequently are cascades.
Waterfalls are valuable today as sources of hydroelectric power.
Angel Falls is the highest waterfall in the world. It is located on Mount Auyantepuli, a plateau in southeastern Venezuela. Angel Fails has an unbroken drop of 2,648 feet and a total height of 3,212 feet. It is more than 1,000 feet higher than any other known falls.
James Angel, an American aviator, discovered Angel Falls in 1935. In 1949, an American expedition explored the canyon and measured the height of the fails. Kukenaam, also in Venezuela, is the second highest waterfall in the world. It measures in at 2,000 feet. Third place honors go to Yosemite National Park's Ribbons Falls at 1,612 feet and close behind at 1,600 feet is Guyana's King George VI Falls.
Especially voluminous or spectacular waterfalls include Tugela in South Africa (1,350 feet), Takakkaw in British Columbia (1,248 feet), Silver Strand in California (1,170) and two in,Switzerland: Staubbach (984 feet) and Trummelbach (950 feet).