Merced River - River Modifications

River Modifications

Despite its partial status as a National Wild and Scenic River, the Merced River has been the subject of dam-building and irrigation diversions. The dams on the Merced River are New Exchequer Dam, McSwain Dam and the Crocker-Huffman Diversion Dam. New Exchequer Dam is the largest dam on the river and forms Lake McClure, which holds 1,032,000 acre feet (1.273×109 m3). This modern structure was preceded by the old Exchequer Dam forming Exchequer Reservoir, with a capacity of 281,000 acre feet (347,000,000 m3). The old concrete arch dam, completed in 1926, has since been inundated by a new rockfill structure.

Many small diversion dams block the Merced River downstream, many of which were built by the Merced Irrigation District to supply water to farms in the Central Valley. The lowermost, the Crocker-Huffman Diversion Dam, was built just before 1907, and completely blocks the passage of anadromous fish up the Merced River. McSwain Dam, the other major lower dam, also serves as a forebay (regulating dam) for New Exchequer Dam. Aside from controlling flows in the lower river the dam also produces some hydroelectricity. Because of the nearly desiccated flow of the river as it nears the mouth, like most other rivers in the San Joaquin Valley, little water in the Merced River actually reaches the Pacific Ocean.

Cascades Diversion Dam was a timber crib dam built in 1917 near where the Merced River flows out of Yosemite Valley. Because of its impact on the environment and damage from floods, the dam, originally built to generate hydropower, was decommissioned in 1985 and removed in the 1990s. The U.S. Bureau of Reclamation surveyed the dam site in 1997 and found that the dam was in danger of failure. Classified as a "high hazard" structure, it was originally considered for inclusion on the National Register of Historic Places but was deemed too dangerous to keep. Today, the Merced to Lake McClure is completely free-flowing and unobstructed by any dams.