Coordinates: 37°20′57″N 120°58′32″W / 37.34917°N 120.97556°W / 37.34917; -120.97556
The Merced River (pronounced /mer-SED/ ), in central California in the United States, flows westwards from the Sierra Nevada to join the San Joaquin River in the southern Central Valley. The river is most famous for flowing through the southern half of Yosemite National Park and the well-known Yosemite Valley.
Originally formed as one of many rivers flowing off the ancestral Sierra Nevada, the most recent Ice Age brought sweeping change to the Merced River. A succession of glaciers carved out the upper basin, and erosion deposited thick layers of rich sediment into the flat floor of the lower Merced River watershed, creating a fertile grassland. Subsequently, the river became habitat for thousands of migrating salmon and hundreds of bird and mammal species. The early people living along the river, including the Miwok, Paiute, and Ahwahneechee, utilized these resources and thrived for hundreds of years on the Merced River.
In the 18th and 19th centuries, Spanish military expeditions travelled into the Central Valley, and the names of many rivers in the southern part of the valley originate from one led by Gabriel Moraga in the early 1800s. The California Gold Rush saw the middle Merced River being extensively mined for gold, and later silver and other minerals. Conflicts between Native Americans and miners caused the creation of the Mariposa Battalion, which forcefully drove the upper basin tribes away from their homelands. A railroad operated along the river for nearly sixty years, shipping lumber to sawmills in towns lower on the river and carrying increasing numbers of tourists to Yosemite Valley, a popular recreational venue since.
With the arrival of the Central Pacific and Southern Pacific Railroads in California, the economy of the lower basin switched to large-scale irrigated agriculture. The 20th century saw much of the lower course of the river impounded in artificial lakes, and many of the original small towns on the river have been abandoned in favor of larger urban centers such as Turlock, Merced and Livingston.
Course
Beginning at the confluence of the Lyell Fork, Triple Peak Fork and Merced Peak Fork in the southeastern corner of Yosemite National Park, near Merced Lake, the Merced River proceeds to flow 112 miles (180 km) westward through a series of canyons, gorges and finally the flat plains of the Central Valley. Its headwaters at 7,917 feet (2,413 m) lie at the foot of the Clark Range, a subrange of the Sierra Nevada. The river flows through a steep-walled granite canyon for nearly 10 miles (16 km) before spilling into Little Yosemite Valley, a glacial valley that precedes the more famous Yosemite Valley. Following the John Muir Trail the Merced River then drops over Nevada Falls and Vernal Falls, receives Illilouette Creek, and passes into the main Yosemite Valley, where it meanders between pine forests that fill the valley floor.
Tenaya, Yosemite, Bridalveil and Pigeon Creeks join the Merced before it breaches the glacial moraine at the valley's end. From there the river picks up Cascade Creek and turns south near El Portal, flowing through Merced River Canyon. State Route 140 follows the river out of the west entrance to the national park, a few miles before the South Fork Merced River, the largest tributary, joins from the left. The river arcs northwest to receive the North Fork, and a few miles after it enters Lake McClure, formed by New Exchequer Dam. The remainder of the river continues to flow west across the Central Valley before joining the San Joaquin River a few miles south of Turlock.
Watershed
The drainage basin of the Merced River is located in the central Sierra Nevada, spreading across 1,726 square miles (4,470 km 2 ). It can be characterized as the slightly smaller southern companion of the Tuolumne River, the major Sierran river just north of the Merced. On the south, the basin borders on the headwaters of the San Joaquin River itself. The Merced River watershed includes the subwatersheds of Illilouette, Echo, Tenaya, Yosemite, Bridalveil, Cascade, and Dry Creeks, as well as the South and North forks of the river, of which the South Fork, at 43 miles (69 km) long, is the largest. Tributaries to the South Fork include Bishop, Rail, Alder and Chilnualna Creeks. There are also many lakes in the watershed of the Merced River, including Merced Lake, Tenaya Lake, the Chain Lakes, May Lake, Lake McClure, and Lake McSwain.
Much of the basin of the Merced River is characterized by an alpine climate. The upper basin of the river receives heavy snowfall in the winter, which is usually enough to feed the river and its tributaries year round. However, most of the snowpack melts by the late autumn, reducing flows in the main stem and its tributaries significantly, and causing some smaller tributaries to dry up altogether. Snowmelt constitutes a majority of the river's springtime and early summer flow, and provides up to 85% of the flow above Happy Isles. The dry season depends more on groundwater to supply the river's flow. The middle and lower basin are dominated by an arid, Mediterranean or even semi-desert climate.
The Merced River is the third largest tributary of the San Joaquin River. Before irrigation started in the Central Valley and dams were constructed, the river's natural flow was much higher than the current average of 203 cubic feet per second (5.7 m 3 /s), or about 1,000,000 acre feet (1.2 × 10 9 m 3 ) per year. Upper on the river, at Happy Isles, the average flow is 355 cubic feet per second (10.1 m 3 /s). The United States Geological Survey has river gauges at three locations along the Merced River: at Happy Isles, Lake McClure, and at the mouth. The first two record flows unaffected by dams and human intervention, but discharge at the mouth is chiefly controlled by New Exchequer Dam. The Lake McClure gauge is probaby the most accurate gauge for flows overall. A peak of 47,700 cubic feet per second (1,350 m 3 /s) was reported there in 1911. For the mouth gauge, the highest flow was only 13,600 cubic feet per second (390 m 3 /s) in 1950. Finally, for the gauge at Happy Isles, the largest flow ever recorded was 10,100 cubic feet per second (290 m 3 /s) in the 1997 Yosemite floods, which destroyed many campgrounds, roads, paths, and bridges in the valley.
The Merced River's primary human use is irrigation. The Merced Irrigation District (MID) operates most of the irrigation infrastructure, which supplies water to about 154,394 acres (624.81 km 2 ) of farmland. As a whole, the system includes about 4,000 sets of control gates and 793 miles (1.276 km 2 ) of canals. Irrigation has taken most of the water out of the lower river, which now rarely reaches the sea. Water that does return to the river does so in for form of irrigation return flows, which carry pesticides, fertilizer and other pollutants. The MID is federally required to allow at least 15,000 acre feet (19,000,000 m 3 ) of water annually to flow continually down the river, not including flooding. The water has allowed the San Joaquin River below the Merced River confluence to contain water, while above the confluence all of its water is usually diverted for irrigation.
Ecology
See also: Ecology of the Sierra NevadaAccording to a study in 2006, there were 37 species of fish, 127 bird species, and 140 insect and invertebrate species found in the Merced River watershed. Most of these species are divided between the upper and lower watershed, which is usually defined by Lake McClure, formed by New Exchequer Dam.
Of the fish, there were 26 species found in the lower Central Valley portion of the river, including Sacramento sucker, smallmouth bass, largemouth bass and carp, all but three of which are resident species, and some of which were intentionally introduced by humans. The three anadromous fish species that still use the lower Merced are the chinook salmon, Pacific lamprey and striped bass. The upper section of the river, defined as the stretch from Lake McClure to the headwaters, had 11 species of fish. Historically, the range of anadromous fish extended to the head of Yosemite Valley, but by the 1950s and 1960s, except in the wettest years, not enough water flowed down the Merced and San Joaquin Rivers at all to allow them to spawn in significant numbers. New water use requirements have allowed many anadromous species to revitalize, from a low of 500 chinook salmon in the mid-20th century to a high of 30,000 fish in 1984. After the 1970s, the annual run was about
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