The Merced River (; Spanish: Río Merced, meaning "Mercy River") is a -long tributary of the San Joaquin River in central California, United States. It flows from the Sierra Nevada into the San Joaquin Valley and is best known for its steep course through the southern part of Yosemite National Park, where it is the primary watercourse in Yosemite Valley. Below the mountains, the river's character changes as it becomes a slow meandering stream across the agricultural San Joaquin Valley.
The river began to take shape as the Sierra Nevada rose about 10 million years ago, and sediment eroded from its canyon helped form the flat floor of the San Joaquin Valley. Glaciation during the ice ages carved the high-elevation portions of the watershed, including Yosemite Valley, into their present form. Historically, an extensive riparian zone provided habitat for millions of migrating birds, and the river supported one of the southernmost runs of chinook salmon in North America.
Miwok and Paiute people lived along the river for thousands of years before Spanish and Mexican military expeditions passed through in the early 19th century. The California Gold Rush brought settlers to towns along the lower Merced River. The Yosemite Valley Railroad was built along the Merced River canyon, enabling mining and logging in the upper watershed and later carrying tourists to Yosemite National Park. Conflicts between settlers and Native Americans led to the expulsion of the Ahwahnechee from Yosemite.
Large-scale irrigation was introduced to the San Joaquin Valley in the late 19th century, leading to construction of numerous dams that blocked migrating salmon and caused steep declines in riparian habitat. Diversion of water for irrigation often reduces the river to a small stream by the time it reaches its mouth. Mitigation efforts include habitat conservation work, re-establishment of historic streamflow patterns, and operation of a salmon hatchery.
Course and watershed
The Merced River is one of 15 major river systems in the Sierra Nevada. Originating in Yosemite’s alpine peaks, the Merced River flows west for to its confluence with the San Joaquin River outside the park in the Central Valley, encompassing a drainage basin of . The first of the Merced River are designated as Wild and Scenic.
The headwaters of the Merced River originate in Yosemite in several watersheds: the Lyell Fork, Triple Peak Fork, Merced Peak Fork, and Red Peak Fork. These watersheds are at the far eastern side of the Merced River watershed. The Tuolumne, Mono, and San Joaquin River watersheds are to the north, east, and south, respectively.
The headwaters of the Merced River lie at at the foot of the Clark Range, a subrange of the Sierra Nevada. The river rises at the confluence of the Triple Peak Fork and Merced Peak Fork after they cascade down glacially polished slopes from the high country in the eastern part of Yosemite National Park. From its headwaters, the main stem of the Merced River flows freely through a wilderness landscape of alpine peaks, glacially carved valleys, and high-elevation meadows.
The river makes a dramatic entry into Yosemite Valley, rushing over towering cliffs in prominent waterfalls. The Merced drops over Nevada Fall and Vernal Fall, together known as the "Giant Staircase", then receives Illilouette Creek and flows into Yosemite Valley, where it meanders between pine forests and meadows on the valley floor.
At the west end of Yosemite Valley, the canyon narrows and the river becomes a cascade of continuous rapids through the Merced Gorge. The gradient changes abruptly at the park boundary, where the river continues through El Portal on its journey through the Sierra Nevada foothills. The river arcs northwest to receive the North Fork, then enters Lake McClure, formed by New Exchequer Dam. Below New Exchequer, the river flows west through a heavily irrigated region of the Central Valley, passing through McSwain and Crocker-Huffman Dams and the cities of Hopeton, Delhi, and Livingston. It joins the San Joaquin River at Hills Ferry, a few miles south of Turlock.
The South Fork Merced River originates at the Sierra crest from the southwestern slopes of Triple Divide Peak and the west-facing slopes of Gale Peak and Sing Peak. The South Fork Merced River flows southwest through Yosemite Wilderness (south of the Clark Range) and the community of Wawona. The South Fork Merced River exits the park less than a mile below the Wawona Campground and then flows through the Sierra National Forest to the confluence with the main stem of the Merced River, west of El Portal.
Hydrology Much of the Merced River basin is at high elevation, where an alpine climate prevails. The Sierra receives heavy snowfall in winter, which melts in spring and early summer, causing annual flooding. By late autumn, the river level has dropped considerably, and some smaller tributaries dry up altogether. Up to 85 percent of the flow above Happy Isles comes from snowmelt. In the dry season, groundwater provides the only base flow. The river is also fed by dozens of high Sierra lakes, the largest of which include Merced Lake, Tenaya Lake, Ostrander Lake, the Chain Lakes, and May Lake. The foothills experience a drier Mediterranean climate, while the San Joaquin Valley floor is dry enough to be classified as semi-arid.
The Merced River is the third-largest tributary of the San Joaquin River. Before irrigation began in the Central Valley and dams were built, the river's natural flow at the mouth was much higher than the current average of , or about per year. Upstream at Happy Isles, the average flow is . The United States Geological Survey (USGS) maintains stream gauges at three locations along the Merced River: at Happy Isles, above Lake McClure, and at the mouth. The first two record flows unaffected by dams, but discharge at the mouth is controlled primarily by New Exchequer Dam. The Lake McClure gauge, at the former mining town of Bagby, is probably the most representative for overall flows. The average annual flow recorded there was from 1923 to 1966. A peak of was recorded there on December 23, 1955. At the mouth gauge, the highest flow was only in 1950. The gauge stopped operation in 1925 and resumed in 1941. From 1996 to 2001 no data was recorded, presumably because of the 1998 El Nino year and the 1997 Yosemite floods. The gauge resumed operation in 2002. As this gauge is below the New Exchequer Dam, the USGS notes that all "Discharge affected by Regulation or Diversion". At Happy Isles, the largest flow ever recorded was during the 1997 Yosemite floods, which destroyed campgrounds, roads, paths, and bridges throughout the valley. The USGS notes that some "Discharge due to Snowmelt, Hurricane, Ice-Jam or Debris Dam breakup" caused several peaks.
Ecology
According to a 2006 study, there were 37 fish species, 127 bird species, and 140 insect and invertebrate species in the Merced River watershed. Differences in climate produce a large disparity between species found in the upper watershed (above Lake McClure) and along the lower river.
The lower Central Valley portion of the river supports 26 species of native and introduced fish, including Sacramento sucker, smallmouth bass, largemouth bass, and carp. Anadromous species in the lower river include chinook salmon, Pacific lamprey, and striped bass. The upper river, from Lake McClure to the headwaters, has 11 fish species. Historically, anadromous fish ranged as far as the waterfalls at the head of Yosemite Valley, but dams have blocked their migration since the early 1900s, and diversions frequently dewater remaining spawning habitat in the lower river. Environmental measures enacted in the late 20th century had some success in boosting chinook salmon populations, from a record low of 500 fish during several years in the 1950s to a high of 30,000 in 1984. Since the 1970s, the annual chinook run has averaged about 5,300.
Of the 127 bird species found along the Merced River, only 35 occur along its entire length. Many are migratory, passing through the area only a few times per year, while 109 species are found only during the breeding season. Birds are more abundant along the slow-moving lower river, which provides more riparian habitat than the rocky, swift upper reaches. Common species throughout the basin include ruby-crowned kinglet, cedar waxwing, American robin, yellow-rumped warbler, tree swallow, and European starling, along with several endangered species, including white-tailed kite and Swainson's hawk. Birds common in the middle and upper sections include mourning dove, Cassin's finch, California quail, dark-eyed junco, woodpecker, dipper, great blue heron, Western scrub jay, red-winged blackbird, red-tailed hawk, turkey vulture, cliff swallow, canyon wren, merganser, and bald eagle. Common insects along the river include mayflies, stoneflies, and caddisflies. The river is affected by invasive Asiatic clam, Chinese mitten crab, and New Zealand mud snail.
Plant species found throughout the middle and upper basin include California poppy, white alder, Oregon ash, oak, poison oak, bigleaf maple, Indian rhubarb, buttonbush, willow, whiteleaf manzanita, and historically, sugar pine, before heavy logging in the late 19th century. Mammals in the middle and upper watershed include squirrels, raccoons, jackrabbits, bats, skunks, beavers, mule deer, coyote, bobcat, and American black bear.
One species of particular interest is the limestone salamander, an extremely rare amphibian whose only habitat is the Merced Canyon downstream of Yosemite Valley. The salamander depends on the limestone walls of the canyon to survive. To protect the species, a segment of the canyon covering was designated an "Area of Critical Environmental Concern" in 1986.
Geology
When the North American Plate on its westward journey encountered the Pacific Plate approximately 250 million years ago during the Paleozoic, the latter began to subduct under the North American continent. Intense subsurface pressure caused portions of the Pacific Plate to melt, and the resulting upwelling magma pushed up and hardened into the granite batholith that makes up much of the Sierra Nevada. Extensive layers of marine sedimentary rock that originally constituted the ancient Pacific seabed were also pushed up by the rising granite, and the ancestral Merced River formed on this rock layer. Over millions of years, the Merced cut a deep canyon through the softer sedimentary rock, eventually reaching the hard granite beneath. Encountering this resistant layer slowed the river's downcutting, although tributary streams continued to widen the ancient canyon.
Over about 80 million years, erosion transported massive amounts of alluvial sediment to the floor of the Central Valley, where it was trapped between the Coast Ranges on the west and the Sierra Nevada on the east, forming a flat and fertile land surface. The present-day form of the upper Merced River watershed was shaped by glaciers, and the lower watershed was indirectly but significantly affected.