The Merced River (; Spanish: Río Merced, meaning "Mercy River") is a 145-mile (233 km)-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.
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