page_title: Merced River
section_title: Ecology
hierarchical_section_title: Merced River / Ecology
caption_attribution_description: Happy Isles, Yosemite National Park
context_page_description: The Merced River, in the central part of the U.S. state of California, is a 145-mile-long tributary of the San Joaquin River flowing from the Sierra Nevada into the San Joaquin Valley. It is most well known for its swift and steep course through the southern part of Yosemite National Park, where it is the primary watercourse flowing through Yosemite Valley. The river's character changes dramatically once it reaches the plains of the agricultural San Joaquin Valley, where it becomes a slow-moving meandering stream.
  The river first formed 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 parts of the watershed, including Yosemite Valley, into their present shape. Historically, there was an extensive riparian zone which provided habitat for millions of migrating birds, and the river had 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.
context_section_description: According to a study in 2006, there were 37 fish species, 127 bird species, and 140 insect and invertebrate species found in the Merced River watershed. Due to differences in climate, there is a large disparity between species found in the upper watershed (above Lake McClure) and along the lower river.
  There are 26 species of native and introduced fish in the lower Central Valley portion of the river, including Sacramento sucker, smallmouth bass, largemouth bass and carp. Anadromous fish species found in the lower river are chinook salmon, Pacific lamprey and striped bass. The upper section of the river, defined as the stretch from Lake McClure to the headwaters, has 11 species of fish. Historically, the range of anadromous fish extended to the waterfalls at the head of Yosemite Valley, but dams have blocked their migration since the early 1900s, and diversions frequently dewater their remaining spawning habitat in the lower part of the river. Environmental measures enacted in the late 20th century have 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 the entire length of the river. Many of these birds are migratory and only pass the area a few times every year, while 109 species of birds are found only in the breeding season. Birds are more abundant along the slow-moving lower river, which has more suitable riparian habitat compared to the rocky, swift upper river. Common species of bird throughout the basin include ruby-crowned kinglet, cedar waxwing, American robin, yellow-rumped warbler, tree swallow and European starling, and several endangered species, including white-tailed kite and Swainson's hawk. Birds that occur commonly in the middle and upper sections of the Merced River include mourning dove, Cassin's finch, California quail, dark-eyed junco, woodpecker, dipper, great blue heron, scrub jay, red-winged blackbird, red-tailed hawk, turkey vulture, cliff swallow, canyon wren, merganser, and bald eagles. Common insects found along the river include mayflies, stoneflies and caddisflies. The river is impacted by invasive Asiatic clam, Chinese mitten crab, and New Zealand mud snail.
  Many species of plants are found throughout the middle and upper basin, including 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. Squirrels, raccoon, jackrabbits, bats, skunks, beavers, mule deer, coyote, bobcat and black bear are among the mammal species found in the middle and upper watershed.
  One species of 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 Merced Canyon to survive. To protect the salamander, a 20-mile (32 km) segment of the canyon covering 1,600 acres (6.5 km²) was designated an "Area of Critical Environmental Concern" in 1986.
caption: Salmon were historically able to swim as far upstream as Happy Isles, seen here, but dams now block them from doing so
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