Yosemite National Park
Water and ice

The Tuolumne and Merced River systems originate along the crest of the Sierra in the park and have carved river canyons 3,000 to 4,000 feet (910 to 1,220 m) deep. The Tuolumne River drains the entire northern portion of the park, an area of approximately 680 square miles (1,800 km). The Merced River begins in the park's southern peaks, primarily the Cathedral and Clark Ranges, and drains an area of approximately 511 square miles (1,320 km).

Hydrologic processes, including glaciation, flooding, and fluvial geomorphic response, have been fundamental in creating park landforms. The park contains approximately 3,200 lakes (greater than 100 m), two reservoirs, and 1,700 miles (2,700 km) of streams. Wetlands flourish in valley bottoms throughout the park, and are often hydrologically linked to nearby lakes and rivers through seasonal flooding and groundwater. Meadow habitats, distributed at elevations from 3,000 to 11,000 feet (910 to 3,350 m) in the park, are generally wetlands, as are the riparian habitats found on the banks of Yosemite's watercourses.

Yosemite is famous for its high concentration of waterfalls in a small area. Numerous sheer drops, glacial steps and hanging valleys in the park feature spectacular cascades, especially during April, May, and June (as the snow melts). Located in Yosemite Valley, Yosemite Falls is the fourth tallest waterfall in North America at 2,425 feet (739 m) according to the World Waterfall Database. Also in the valley is the much lower volume Ribbon Falls, which has the highest single vertical drop, 1,612 feet (491 m). Perhaps the most prominent of the valley waterfalls is Bridalveil Fall. Wapama Falls in Hetch Hetchy Valley is another notable waterfall. Hundreds of ephemeral waterfalls become active in the park after heavy rains or melting snowpack.

Park glaciers are relatively small and occupy areas that are in almost permanent shade, such as north- and northeast-facing cirques. Lyell Glacier is the largest glacier in Yosemite (the Palisades Glaciers are the largest in the Sierra Nevada) and covers 160 acres (65 ha). None of the Yosemite glaciers are a remnant of the Ice Age alpine glaciers responsible for sculpting the Yosemite landscape. Instead, they were formed during one of the neoglacial episodes that have occurred since the thawing of the Ice Age (such as the Little Ice Age). Many Yosemite glaciers have disappeared, such as the Black Mountain Glacier that was marked in 1871 and had gone by the mid-1980s. Yosemite's final two glaciers – the Lyell and Maclure glaciers – have receded over the last 100 years and are expected to disappear as a result of climate change.
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