Yosemite National Park is a national park located in the eastern portions of Tuolumne, Mariposa and Madera counties in east central California, United States. The park covers an area of 761,266 acres or 1,189 square miles (3,081 km²) and reaches across the western slopes of the Sierra Nevada mountain chain. Yosemite is visited by over 3.5 million people each year, many of whom only spend time in the seven square miles (18 km²) of Yosemite Valley. Designated a World Heritage Site in 1984, Yosemite is internationally recognized for its spectacular granite cliffs, waterfalls, clear streams, Giant Sequoia groves, and biological diversity. Almost 95% of the park is designated wilderness. Although not the first designated national park, Yosemite was a focal point in the development of the national park idea, largely owing to the work of people like John Muir and Galen Clark.

Yosemite is one of the largest and least fragmented habitat blocks in the Sierra Nevada, and the park supports a diversity of plants and animals. The park has an elevation range from 2,000 to 13,114 feet (600 to 4,000 m) and contains five major vegetation zones: chaparral/oak woodland, lower montane, upper montane, subalpine, and alpine. Of California's 7,000 plant species, about 50% occur in the Sierra Nevada and more than 20% within Yosemite. There is suitable habitat or documentation for more than 160 rare plants in the park, with rare local geologic formations and unique soils characterizing the restricted ranges many of these plants occupy.

The geology of the Yosemite area is characterized by granitic rocks and remnants of older rock. About 10 million years ago, the Sierra Nevada was uplifted and then tilted to form its relatively gentle western slopes and the more dramatic eastern slopes. The uplift increased the steepness of stream and river beds, resulting in formation of deep, narrow canyons. About 1 million years ago, snow and ice accumulated, forming glaciers at the higher alpine meadows that moved down the river valleys. Ice thickness in Yosemite Valley may have reached 4,000 feet (1200 m) during the early glacial episode. The downslope movement of the ice masses cut and sculpted the U-shaped valley that attracts so many visitors to its scenic vistas today.

Geography

Yosemite National Park is located in the central Sierra Nevada of California. It takes approximately 3.5 hours to drive to the park from San Francisco and approximately 6 hours from Los Angeles. Yosemite is surrounded by wilderness areas: the Ansel Adams Wilderness to the southeast, the Hoover Wilderness to the northeast, and the Emigrant Wilderness to the north.

The 1,189 sq mi (3,081 km²) park is roughly the size of Rhode Island and contains thousands of lakes and ponds, 1,600 miles (2,600 km) of streams, 800 miles (1300 km) of hiking trails, and 350 miles (560 km) of roads. Two federally designated Wild and Scenic Rivers, the Merced and the Tuolumne, begin within Yosemite's borders and flow westward through the Sierra foothills, into the Central Valley of California. Annual park visitation exceeds 3.5 million, with most visitor use concentrated in the seven square mile (18 km²) area of Yosemite Valley.

Rocks and erosion

Almost all of the landforms in the Yosemite area are cut from the granitic rock of the Sierra Nevada Batholith (a batholith is a large mass of intrusive igneous rock that formed deep below the surface). About 5% of the park (mostly in its eastern margin near Mount Dana) are from metamorphosed volcanic and sedimentary rocks. These rocks are called roof pendants because they were once the roof of the underlying granitic rock.

Erosion acting upon different types of uplift-created joint and fracture systems is responsible for creating the valleys, canyons, domes, and other features we see today. These joints and fracture systems do not move, and are therefore not faults. Spacing between joints is controlled by the amount of silica in the granite and granodiorite rocks; more silica tends to create a more resistant rock, resulting in larger spaces between joints and fractures.

Pillars and columns, such as Washington Column and Lost Arrow, are created by cross joints. Erosion acting on master joints is responsible for creating valleys and later canyons. The single most erosive force over the last few million years has been large alpine glaciers, which have turned the previously V-shaped river-cut valleys into U-shaped glacial-cut canyons. Exfoliation (caused by the tendency of crystals in plutonic rocks to expand at the surface) acting on granitic rock with widely spaced joints is responsible for creating domes such as Half Dome and North Dome and inset arches like Royal Arches.

Popular features

Yosemite Valley represents only one percent of the park area, but this is where most visitors arrive and stay. El Capitan, a prominent granite cliff that looms over the valley, is one of the most popular rock climbing destinations in the world because of its diverse range of climbing routes in addition to its year-round accessibility. Granite domes such as Sentinel Rock and Half Dome rise 3,000 feet and 4,800 feet (900 and 1,450 m), respectively, above the valley floor.

The high country of Yosemite contains beautiful areas such as Tuolumne Meadows, Dana Meadows, the Clark Range, the Cathedral Range, and the Kuna Crest. The Sierra crest and the Pacific Crest Trail run through Yosemite, with peaks of red metamorphic rock, such as Mount Dana and Mount Gibbs, and granite peaks, such as Mount Conness. Mount Lyell is the highest point in the park.

The park has three groves of ancient Giant Sequoia trees; the Mariposa Grove (200 trees), the Tuolumne Grove (25 trees), and the Merced Grove (20 trees). This species grows larger in volume than any other and is one of the tallest and longest-lived. These trees were much more widespread before the start of the last Ice Age.

Water and ice

Tuolumne and Merced River systems originate along the crest of the Sierra Nevada in the park and have carved river canyons 3,000 to 4,000 feet (900 to 1,200 m) deep. The Tuolumne River drains the entire northern portion of the park, an area of approximately 680 square miles (1,760 km²). 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 km²).

Hydrologic processes, including glaciation, flooding, and fluvial geomorphic response, have been fundamental in creating landforms in the park. The park also contains approximately 3,200 lakes (greater than 100 m²), two reservoirs, and 1,700 miles (2,700 km) of streams, all of which help form these two large watersheds. Wetlands in Yosemite occur in valley bottoms throughout the park, and are often hydrologically linked to nearby lakes and rivers through seasonal flooding and groundwater movement. Meadow habitats, distributed at elevations from 3,000 to 11,000 feet (900 to 3,500 m) in the park, are generally wetlands, as are the riparian habitats found on the banks of Yosemite's numerous streams and rivers.

Yosemite is famous for its high concentration of waterfalls in a small area. Numerous sheer drops, glacial steps and hanging valleys in the park provide many places for waterfalls to exist, especially during April, May, and June (the snowmelt season). Located in Yosemite Valley, the 2,425-foot-high (739 m) Yosemite Falls is the highest in North America. Also in the valley is the much lower volume Ribbon Falls, which has the highest single vertical drop, 1,612 feet (492 m). Perhaps the most prominent of the Yosemite waterfalls is Bridalveil Fall, which is the waterfall seen from the Tunnel View viewpoint at the east end of the Wawona Tunnel. Wapama Falls in Hetch Hetchy Valley is another notable waterfall. Hundreds of ephemeral waterfalls also exist in the park.

All glaciers in the park are relatively small glaciers that occupy areas that are in almost permanent shade, such as north- and northeast-facing cirques. Lyell Glacier is the largest glacier in the Sierra Nevada (and therefore the park) and covers 160 acres (65 ha). None of the Yosemite glaciers are a remnant of the much, much larger 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). Global warming has reduced the number and size of glaciers around the world. Many Yosemite glaciers, including Merced Glacier, which was discovered by John Muir in 1871 and bolstered his glacial origins theory of the Yosemite area, have disappeared and most of the others have lost up to 75% of their surface area.

Climate

Yosemite has a Mediterranean climate, meaning most precipitation falls during the mild winter, and the other seasons are nearly dry (less than 3% of precipitation falls during the long, hot summers). Due to orographic lift, precipitation increases with elevation up to 8,000 feet (2,400 m) where it slowly decreases to the crest. Precipitation amounts vary from 36 inches (915 mm) at 4,000 feet (1,200 m) elevation to 50 inches (1,200 mm) at 8,600 feet (2,600 m). Snow does not typically persist on the ground until November in the high country. It accumulates all winter and into March or early April.

Temperature decreases with increasing elevation. Temperature extremes are moderated by the fact that Yosemite is only about 100 miles (160 km) from the Pacific Ocean. An anticyclone sits off the coast of California in the summer, sending cool air masses toward the Sierra Nevada that result in clean dry air in the Yosemite area.

Mean daily temperatures range from 25 to 53 °F (-3.9 to 11.5 °C) at Tuolumne Meadows at 8,600 feet (2,600 m). At the Wawona Entrance (elevation 5,130 feet; 1,564 m), mean daily temperature ranges from 36 to 67 °F (2.2 to 19.4 °C). At the lower elevations below 5,000 feet (1525 m), temperatures are hotter; the mean daily high temperature at Yosemite Valley (elevation 3,966 feet; 1,209 m) varies from 46 to 90 °F (7.8 to 32.2 °C). At elevations above 8,000 feet (2,440 m), the hot, dry summer temperatures are moderated by frequent summer thunderstorms, along with snow that can persist into July. The combination of dry vegetation, low relative humidity, and thunderstorms results in frequent lightning-caused fires as well.

At the park headquarters, normal January temperatures range from 47.2°F to 25.6°F and normal July temperatures range from 89.9°F to 53.2°F. There are an average of 47.7 days with highs of 90°F (32°C) or higher and an average of 137.5 days with lows of 32°F (0°C) or lower. Freezing temperatures have been recorded in every month of the year. The record high temperature was 115°F on July 20, 1915. The record low temperature was -6°F on January 2, 1924. Average annual precipitation is 36.57 inches; there an average of 69 days with measurable precipitation. The wettest year was 1983 with 68.94 inches and the dryest year was 1976 with 14.84 inches. The most precipitation in one month was 29.61 inches in December 1955 and the most in one day was 6.92 inches on December 23, 1955. Average annual snowfall is 65.6 inches. The snowiest year was 1967 with 154.9 inches. The most snow in one month was 140.8 inches in January 1993.

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