New Don Pedro Dam

New Don Pedro Dam is a 585ft tall earthen embankment dam that spans the Tuolumne River and is situated in Tuolumne County in the US State of California. After four years of construction, the dam was finished in 1971 and replaced the 1924 concrete-arch old Don Pedro Dam. The New Don Pedro Dam is the country's ninth-tallest dam

About The Dam

USGS image of the New Don Pedro Dam
USGS image of the New Don Pedro Dam. Image Credit: USGS, Public domain, via Wikimedia Commons

This gravity dam made of concrete, soil, and rock, which impounds Don Pedro Reservoir in the Sierra Nevada foothills, is primarily used for agricultural water storage, flood control, and hydroelectricity generation. The Modesto Irrigation District (MID) and the Turlock Irrigation District (TID) own and operate the New Don Pedro Dam. The original dam was named after the Tuolumne River mining town of Don Pedros Bar, which was named after prospector Pierre "Don Pedro" Sainsevain. The Don Pedro Reservoir formed by the New Don Pedro Dam is California's sixth-largest artificial reservoir. The maximum storage capacity of the New Don Pedro Reservoir is 2,030,000 acre-feet. The downstream channel capacity is rated at 15,000 cubic feet per second of discharge flow. The system is intended to safeguard homes living along the Tuolumne River downstream of Don Pedro.

Construction

Lake Don Pedro and the surrounding Sierra Nevada Mountains, California
Lake Don Pedro and the surrounding Sierra Nevada Mountains, California. 

The first Don Pedro Dam was constructed in 1923 by the Turlock and Modesto Irrigation Districts (TID and MID). With a storage capacity of 289,000 acres, it could barely meet the irrigation demands of a single growing season. Following many dry winters, the districts agreed to replace the old dam with a much bigger one to store enough water to span several dry years. San Francisco collaborated with the two irrigation agencies to build what was known at the time as the New Don Pedro Project. After the old dam was flooded by the waters of the larger reservoir, the 'New' was legally omitted from its name. It is today referred to as the Don Pedro Reservoir, Don Pedro Lake, or Lake Don Pedro.

San Francisco received relief from upstream flood management responsibilities on the Tuolumne River in exchange for its financial contribution to the Project's construction, as well as increased freedom in its upstream operations. San Francisco may pre-release water into Don Pedro through a complex banking and accounting arrangement, where it becomes the possession of the districts. San Francisco may therefore redirect upstream flows to which the districts would otherwise be entitled with a "credit" in the Don Pedro water bank. This approach serves to save Tuolumne River's water and optimize its useful usage. San Francisco does not own any of the Don Pedro Project or the reservoir's water. It also does not physically transfer any of the reservoir's water into its own water system. The New Don Pedro Project began construction in 1967 and was finished four years later at the cost of $105 million. The reservoir, which was built primarily to hold agricultural water, has a capacity of 2,030,000 acre-feet and provides several other services, including electricity generation, flood control, and recreation. The ancient dam is still on site, 250 feet below the reservoir's entire surface.

Power Plant

TID has been distributing power to retail clients as a public power agency since 1923. TID owns and operates the reservoir and power-producing facilities. Don Pedro's power plant takes up the whole width of the river channel at the dam's toe. It's an outdoor building made of reinforced concrete that is used to house three 45,500 kilowatt generating units powered by three 70,000 horsepower turbines. A fourth unit was built in 1989, and the facility now operates three 55,000 kW and one 38,000 kW generator, each with 85,000 horsepower, for a total capacity of 203 megawatts, with 139 MW flowing to TID and 64 MW going to MID.

Dam Safety

At the very least, the New Don Pedro Dam and its associated structures are inspected by:

  • The Federal Energy Regulatory Commission, as well as the California Department of Water Resources Division of Dam Safety, conduct annual dam safety inspections. 
  • TID
  • External consultants as needed

TID conducted a focused study of the Don Pedro spillway structures in 2017, following the Oroville Spillway Incident. An examination of the original design and construction records, extensive inspections and testing, and an analysis of probable failure mechanisms were all part of the evaluation. The evaluation was led by experienced external experts and included both FERC and DSOD personnel. There were no safety problems discovered, and the spillway structures are deemed safe for operation.

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