The Merced River () is a -long
The headwaters of the main stem 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. 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.
thumb|left|upright|alt=Steep walls of the Merced River canyon seen from above.|The Merced River canyon in Yosemite National Park
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 foothills experience a drier Mediterranean climate, while the San Joaquin Valley floor is dry enough to be classified as semi-arid.
thumb|right|alt=Clear alpine lake surrounded by granite peaks in Yosemite.|May Lake, at the headwaters of Snow Creek, is one of many lakes feeding the Merced River.
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 . A peak of was recorded there on December 23, 1955. At the mouth gauge, the highest flow was only in 1950. At Happy Isles, the largest flow ever recorded was during the 1997 Yosemite floods, which destroyed campgrounds, roads, paths, and bridges throughout the valley.
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.
thumb|left|alt=Turbulent whitewater at Happy Isles during a flood event.|Salmon historically migrated as far upstream as [[Happy Isles, but dams now block their passage.]]
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.
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.
Geology
thumb|upright|right|alt=Two waterfalls descending a granite staircase between Yosemite and Little Yosemite Valley.|The Giant Staircase between Yosemite and Little Yosemite Valley
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.
When the last glacial period arrived, a series of four large valley glaciers filled the upper basin of the Merced River. These glaciers rose in branches upstream of Yosemite Valley, descending from the Merced River headwaters, Tenaya Canyon, and Illilouette Creek. Tenaya Canyon was actually eroded even deeper by an arm of the Tuolumne Glacier, which formed the Grand Canyon of the Tuolumne and Hetch Hetchy Valley on the Tuolumne River to the north. Little Yosemite Valley formed where the underlying rock was harder than that below the Giant Staircase, the cliff wall containing Vernal Fall and Nevada Fall. The three branches of each glacier combined to form a single glacier about thick at maximum, stretching downstream past the mouth of Yosemite Valley well into Merced Canyon. These glaciers formed the granite cliffs now known as Half Dome, El Capitan, and Clouds Rest.
The first and largest glacier was the Sherwin (or pre-Tahoe) glacier, which eroded the upper Merced watershed to an extent close to its present form. Three stages followed during the Wisconsinian glaciation: the Tahoe, Tenaya, and Tioga stages, of which the Tioga was the smallest. The Tioga glacier left a rocky moraine at the mouth of Yosemite Valley. This moraine was one of several deposited by the four glaciations, which also include Medial Moraine and Bridalveil Moraine. After the Tioga Glacier retreated, the moraine impounded a lake that flooded nearly the entire valley. Gradual sedimentation filled Lake Yosemite, creating a broad, flat valley floor. Sediments of glacial origin continued to travel down the Merced, contributing to the flat floor of the Central Valley. Paiute people, originally from the eastern Sierra near Mono Lake, also lived in the upper watershed. The Sierra Miwok and Mono Lake Paiute, through cultural interaction over time, formed a distinct group, the Ahwahnechee, whose name derives from Ahwahnee, meaning "the valley shaped like a big mouth"a reference to the U-shaped Yosemite Valley.
In the early 19th century, several military expeditions sent by Spanish colonists from coastal California traveled into the Central Valley. One, headed by lieutenant Gabriel Moraga, arrived on the south bank of the Merced River on September 29, 1806. They named the river Río de Nuestra Señora de la Merced (River of Our Lady of Mercy); the Virgin of Mercy is the patron saint of the diocese of Barcelona, celebrated on September 24. Another expedition in 1805 similarly named the Kings River upon reaching it on January 6, the feast of the Epiphany. Moraga's expedition was part of a series of exploratory ventures, funded by the Spanish government, to find suitable sites for missions in the Central Valley and the Sierra Nevada foothills. In 1808 and 1810, Moraga led further expeditions along the lower Merced River below Merced Canyon, each time without result. Plans to establish a mission chain in the valley were eventually abandoned. In 1855, Merced County was created, named after the river.
Following the establishment of Merced County and California's independence from Mexico, settlers came to the Merced River area and founded small towns. One of the first was Dover, established in 1844 at the confluence of the Merced with the San Joaquin. Dover functioned as an inland port where boats delivered supplies from the San Francisco Bay area to settlers in the San Joaquin Valley. Other towns followed, including Hopeton, Snelling, and Merced Falls, the last named for a set of rapids on the Merced near the present-day site of McSwain Dam. In the late 1880s, a flour mill, woolen mill, and several lumber mills were built at Merced Falls. The Sugar Pine Lumber Company and Yosemite Lumber Company operated mills there for over thirty years, relying on narrow-gauge railroads to ship lumber from the Sierra Nevada along the Merced River. Following construction of the Central Pacific Railroad and Southern Pacific Railroad, many of the river towns were deserted. Several cities that did achieve prominence include Merced and Turlock, both located on the railroad.
thumb|left|upright|alt=Nineteenth-century oil painting of the Merced River flowing through Yosemite Valley with dramatic cliffs.|The Merced River, Yosemite, California, by [[George Henry Burgess]]
The California Gold Rush in the 1850s brought increasing mining activity to Merced Canyon and Yosemite Valley. Many Native Americans in the area revolted, leading to armed conflict between miners and the Ahwahnechee. In 1851, the Mariposa Battalion was formed to drive the remaining Ahwahnechee out of the valley into reservations. The Battalion fought an Ahwahnechee group led by Chief Tenaya over the South Fork of the Merced River. They succeeded in removing most of the Ahwahnechee from Yosemite Valley, first to a reservation near Fresno. Following the Gold Rush, the Ahwahnechee were allowed to return, but further incidents prompted a second battalion to drive them out again, this time to the Mono Lake area.
Even before the establishment of Yosemite National Park, tourists began traveling into Merced Canyon and Yosemite Valley as early as 1855. The first roads into Yosemite Valley were built in the 1870s. The first was the Coulterville Road, originating at Coulterville, followed by the Big Oak Flat Road, a trading route from Stockton to Merced Canyon. Environmental advocacy led by John Muir and Robert Underwood Johnson convinced the U.S. Congress to establish Yosemite National Park in 1890.
thumb|right|alt=Steam locomotive crossing a steel bridge over a river canyon in 1926.|The first passenger train of the [[Yosemite Valley Railroad crossing Barrett Bridge in April 1926]]
The Yosemite Valley Railroad, originally built to serve mineral deposits in Yosemite Valley and Merced Canyon, continued operating through the early 20th century carrying tourists to Yosemite along the Merced River. El Portal Road, constructed through Merced Canyon in 1926, ended passenger service on the railway, but freight operations continued until the mid-1940s, when major flooding destroyed sections of the line.
In the early 20th century, while the upper Merced basin lay mostly protected, the lower river became the focus of dam-building and irrigation diversions by the Merced Irrigation District. The District built the Exchequer Dam, completed in the mid-1920s and raised in the 1960s, as a water storage facility on the Merced River.
Irrigation using water from the Merced River grew substantially until most of the arable land around the riversome was under cultivation. By the late 1950s and early 1960s, irrigation in the San Joaquin Valley was so extensive that many rivers ran dry in sections. Upstream of the Merced's confluence with the San Joaquin, the latter river was usually dry, regaining flow only where the Merced entered. In the mid-20th century, diminished flow in the Merced River meant very few salmon returned to spawn in the lower section. In 1991, the Merced River Hatchery was built beside the river just downstream of the Crocker-Huffman Diversion Dam, the lowermost dam on the Merced. Fall chinook salmon travel up a fish ladder into the hatchery's pools, which are supplied with water diverted from the river.
Yosemite Valley sustained significant damage when the river flooded between December 31, 1996, to January 5, 1997. The flood was the worst in park history. The Merced River at Happy Isles peaked at 10,100 cubic feet per second during the flood. Total park damages were estimated at $178 million.
In 2024, The New York Times reported that farmers had drained significant volumes of water during the 2022 drought, leaving part of the Merced dry without authorities being aware.
River modifications
thumb|right|alt=Small timber dam across a rocky river channel surrounded by forest.|Cascades Diversion Dam before its removal in the 1990s
Despite its partial status as a National Wild and Scenic River, the Merced River has many dams and irrigation diversions. New Exchequer Dam is the largest dam on the river and forms Lake McClure, which holds of water for irrigation, flood control, and hydropower generation. This structure was preceded by the old Exchequer Dam, which formed a reservoir. The old concrete arch dam, completed in 1926, was flooded when the new concrete-face rockfill dam was built in 1967, but it occasionally reappears during periods of low water.
thumb|left|upright|alt=Aerial view of Lake McClure and the Merced River canyon looking into afternoon sun.|Aerial view of [[Lake McClure and the Merced River]]
Downstream of New Exchequer Dam is McSwain Dam, which serves as a regulating forebay for New Exchequer and also generates power. Below McSwain is Merced Falls Dam, an irrigation diversion dam. The lowermost dam, the Crocker-Huffman Diversion Dam, was built in 1906 and completely blocks the passage of anadromous fish up the Merced River. Together, these diversions remove almost half the river's natural flow, and an even greater proportion during dry years.
Cascades Diversion Dam was a timber crib dam built in 1917 near where the Merced flows out of Yosemite Valley. Originally built to generate hydropower, the dam was decommissioned in 1985 but remained standing for years afterward. After the 1997 flood, the U.S. Bureau of Reclamation surveyed the dam and found it in danger of failure. Classified as a "high hazard" structure, it was considered for inclusion on the National Register of Historic Places but was deemed too dangerous, and was subsequently removed. Today, the Merced River above Lake McClure is completely free-flowing and unobstructed by any dams.
The Merced Irrigation District (MID) operates most of the irrigation infrastructure on the lower river, supplying water to of farmland. The system includes about 4,000 control gates and of canals and laterals. Irrigation has removed most of the water from the lower river, which is often reduced to a small stream at its confluence with the San Joaquin. Irrigation return flows carry pesticides, fertilizer, and other pollutants into the river. The MID is federally required to allow at least of water annually to flow down the river, not including floodwater releases.
Recreation
thumb|right|upright|alt=View up the Merced River toward granite cliffs in Yosemite National Park.|The Merced River in a stretch popular for boating and [[whitewater rafting in Yosemite]]
The Merced River and its tributaries are a popular recreational area, in part because of Yosemite National Park. Activities within the watershed include boating, fishing, camping, and hiking. Whitewater rafting is permitted throughout Merced River Canyon from the downstream half of Yosemite Valley to the entrance of Lake McClure. The most difficult rapids in this segment rate Class III and Class IV, mostly upstream of El Portal. Boating is also available on Lake McClure. Camping throughout the upper Merced watershed is generally permitted only in designated campgrounds. Campgrounds along the Merced River and its tributaries include sites at Railroad Flat, McCabe Flat, Willow Placer, Merced Lake, Vogelsang Lake, Sunrise Creek, May Lake, Bridalveil Creek, and a ski hut at Ostrander Lake, the source of Bridalveil Creek.
The name "Railroad Flat" comes from the Yosemite Valley Railroad, which once ran up Merced River Canyon into Yosemite Valley. The old railroad grade still exists and is now the site of a public trail. Many other trails lead throughout the Merced River watershed, notably the John Muir Trail, which starts near Happy Isles and climbs the Giant Staircase past Vernal and Nevada Falls into Little Yosemite Valley and north along Sunrise Creek to join the Pacific Crest Trail near Tuolumne Meadows. Trails also follow the river through Little Yosemite Valley to the headwaters, and along Illilouette, Bridalveil, Yosemite, Alder, and Chilnualna Creeks, and the lower South Fork of the Merced River. Some segments have no trails, including the lower Bridalveil Creek, the upper South Fork, and Tenaya Canyon, which is extremely dangerous.