New Exchequer Dam

New Exchequer Dam is a concrete–faced, rock-fill dam on the Merced River in central California in the United States. It forms Lake McClure, which impounds the river for irrigation and hydroelectric power production and has a capacity of more than 1000000 acre feet. The Merced Irrigation District (MID) operates the dam and was also responsible for its construction.

Built between 1964 and 1967, the dam replaced the old arch type Exchequer Dam and stands 490 ft high. At the time of completion, it was the largest dam of its kind in the world. The dam is named for the town of Exchequer which now lies under the reservoir, while the reservoir is named for Wilbur F. McClure, the State Engineer of California during construction.

History
In 1926, after five years of planning and construction, MID completed the Exchequer or "Great Exchequer" Dam across the Merced River six miles (9.7 km) above the town of Merced Falls. The dam was a concrete gravity–arch structure 326 ft high, holding a 14 mi lake with a capacity of 281000 acre feet of water. Although the dam was to serve primarily for irrigation, power production began ceremoniously on June 23, 1926 with a press of a telegraph key by President Calvin Coolidge, starting the turbines at a 31 megawatt hydroelectric plant. By the 1950s, it became apparent that the limited storage capacity at Exchequer was no longer enough to serve the needs of farmers in the Merced River valley. A high dam was proposed to be built just downstream, creating a reservoir nearly four times the size of Exchequer.

Construction of New Exchequer Dam began on July 8, 1964, directly downstream from the old concrete arch dam. Tudor Engineering Company of San Francisco was responsible for the design of the new dam. The dam wall was constructed in vertical zones, which consisted of compacted, alternating layers of coarse and fine material ranging in thickness from 1.6 to 66 ft. The old Exchequer Dam was incorporated as an upstream toe to help support the rock-fill embankment, which was then armored with a layer of reinforced concrete. The dam was topped out in early 1967 and the power plant went into commercial operation by July.

As the new reservoir filled, it inundated an additional 15 mi of the Merced River canyon and buried sections of the historic Yosemite Valley Railroad and the mining town of Bagby under 50 ft of water. New Exchequer was among the first high concrete–faced rock-fill dams in the world, and its untested design resulted in significant leakage, sometimes up to 100 cuft/s. MID began to repair the leaks in the fall of 1985 under orders from the California Division of Safety of Dams.

The dam has been able to halt major floods in many instances, such as the New Year's Day Flood of 1997. However, it has not always been able to weather the worst droughts – such as in 1977, when the reservoir fell to just 75000 acre feet, a fourteenth of capacity, and in 1991, which saw historic low water levels of 66100 acre feet. In February 2015, the reservoir reached its lowest level on record, at 63489 acre feet or less than 7 percent of total capacity, due to three years of persistent drought.

Dimensions and usage
New Exchequer Dam stands 490 ft high from the foundations and 479 ft above the Merced River. The dam is 1220 ft long, 18 ft wide at the crest, 1000 ft wide at the base and is composed of 5169000 yd3 of fill. High water releases are controlled by an ogee-type, gated overflow spillway located about 2 mi north of the dam. The dam's power station has a capacity of 94.5 megawatts and generates about 316 million kilowatt hours annually.

The reservoir has a storage capacity of 1024600 acre feet, of which 350000 acre feet is reserved for flood control. At full pool, the reservoir has an elevation of 867 ft, with 7147 acre of water and 82 mi of shoreline. To fulfill downstream flood control requirements, the reservoir will only be allowed to rise into the flood-control pool if the flow downstream at Stevinson is forecast to exceed 6000 cuft/s.

MID has proposed raising the spillway gates of the dam, which would provide up to 70000 acre.ft of additional storage. However, this has met with controversy because it would result in part-time flooding of a portion of the Merced River designated Wild and Scenic. Opponents also point out that the raise is unnecessary, since the reservoir has never overflowed due to flooding since its completion in the mid-1960s.

Lake McClure is also extensively developed for recreational activities, with 515 campsites, four boat ramps and two marinas. In 1992, the lake received 606,000 visitor-days, mostly from May to September.