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Small dam on brink of failure amid California’s most powerful storm of the season

A flash flood warning was issued for Tuolumne County on Thursday afternoon as a small dam just outside Yosemite National Park teetered on the brink of failure, authorities said.

Engineers detected “seepage” in Moccasin Dam and alerted the California Department of Transportation and the Department of Water Resources, said DWR spokesman Chris Orrock.

The dam holds less than 600 acre-feet of water. If the water were to rise quickly, causing an overflow, it would rush down Moccasin Creek and portions of highways 120 and 49 until it reached the Don Pedro Reservoir about a mile downstream.

That reservoir has more than enough room to take in the additional water, Orrock said.

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The public was not considered to be in danger. But the threat of an overflow has forced authorities to evacuate the Moccasin Creek fish hatchery and a nearby campground. The dam is part of the Hetch Hetchy Project and generates power for the San Francisco Public Utilities Commission.

“Once emergency responders have assessed and contained the issue from a public safety perspective, DWR’s Division of Safety of Dams will be on site to provide any necessary technical expertise,” the agency said in a statement.

joseph.serna@latimes.com

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