Waterfall

I

Introduction

Waterfall, sudden sheer descent of a stream or river over a steep drop in its bed, sometimes in a free fall. 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 streambed. The highest waterfalls frequently are cascades. Waterfalls are valuable today as sources of hydroelectric power.

II

How Waterfalls Are Formed

Waterfalls develop in several ways. Different rates of erosion where a resistant layer of rock in a streambed overlies softer layers is the principal manner. Subsequent erosion of the softer rock by the falling water undermines and periodically breaks off portions of the harder cap rock. Some of the largest cataracts in the world, Niagara Falls in North America and Victoria Falls in Africa, for example, originated in 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. Bridalveil, Ribbon, and Upper and Lower Yosemite falls in Yosemite National Park, Multnomah Falls (189 m/620 ft) on a minor tributary of the Columbia River in Oregon, Gavarnie (422 m/1385 ft) in France, Sutherland Falls (580 m/1904 ft) in New Zealand, and Staubbach Falls in Switzerland, for instance, all descend from hanging valleys.

Other waterfalls originate where a 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 of the rock bed above the falls move many waterfalls upstream; these ultimately diminish in size, dwindle to rapids, then disappear.

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III

Famous Falls

Especially voluminous or spectacular waterfalls not previously mentioned include Angel Falls (979 m/3212 ft) in Venezuela; Thukela (Tugela) (948 m/3110 ft) in the province of KwaZulu-Natal, South Africa; Cuquenán (610 m/2000 ft) in Venezuela; Takakkaw (503 m/1650 ft) in British Columbia; Great Fall (488 m/1600 ft) in Guyana; Krimmler (381 m/1250 ft) in Austria; Silver Strand Falls (357 m/1170 ft) in California; Wollomombi (335 m/1100 ft) in Australia; Jog Falls (253 m/830 ft) in India; and Iguaçu Falls. The given length measurements for some of these falls involve more than one leap.