After Fears of Collapse, S.F-Owned Dam Said Not to Be in Immediate Danger

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Moccasin Reservoir, just above Lake Don Pedro along the Tuolumne River, was brim full after heavy rain Thursday. The National Weather Service issued a flash-flood warning saying Moccasin Dam was near collapse (Tuolumne County Sheriff's Office via Twitter)

Update, 12:15 a.m. Friday:

San Francisco utility officials and authorities in Tuolumne County say they're closely monitoring conditions at a city-owned dam in the Sierra foothills after a deluge Thursday touched off a surge of floodwater and debris into the reservoir behind the structure.

Moccasin Dam, near the junction of Highways 49 and 120 about 40 miles northeast of Modesto, became a focus of concern early Thursday afternoon after the National Weather Service in Sacramento issued a flash-flood warning based on concern that failure of the 60-foot high structure was "imminent."

That warning was issued as a torrent of muddy water roared down an emergency spillway on one side of the dam.

The dam, along with its reservoir and an adjacent powerhouse, are all part of the San Francisco's Hetchy Hetchy water and power system, which serves 2.5 million people in the Bay Area.

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The San Francisco Public Utilities Commission, which owns and operates the Moccasin facilities, said later Thursday afternoon that as rains and flows into the reservoir eased, it considered a dam failure unlikely. The agency said its crews were working to empty the reservoir, which in normal operating conditions receives water that has run through the powerhouse and discharges it into a tunnel heading for the Bay Area.

NWS Sacramento said in a followup advisory that the potential for a breach continued and its flash-flood warning for the area remained in place.

Moccasin Reservoir has a maximum capacity of 554 acre-feet of water -- about 180 million gallons. A dam collapse would send a surge of water through the fish hatchery and down down Moccasin Creek into the giant Don Pedro Reservoir just downstream.

A 2014 biannual SFPUC report on the Hetch Hetchy system said the dam, built in 1930, was due for a "formal condition assessment" to be completed in July 2017.

The agency's next biannual report, issued in 2016, said "regular inspection of Moccasin Dam has not revealed a need for capital work." The document adds that a formal condition assessment was due to be undertaken -- with the date stated as either July 2020 or sometime in fiscal 2024.

In a summary of physical assessment of the Hetch Hetchy system's dams, tunnels and other infrastructure, the 2016 report said previous assessment reports for Moccasin Dam were not available. The date of its last assessment was likewise listed as unavailable.

The facility falls under the jurisdiction of the state Department of Water Resources Division of Safety of Dams. DSOD's reports on Moccasin Dam were not readily available, though the SFPUC's 2016 report says the Hetch Hetchy Water and Power's system's evaluation of the dam showed it was safe for continued use.

The SFPUC said in a press release that as water surged into Moccasin Reservoir early Thursday afternoon, safety personnel saw water leaking from the dam. Warnings were issued and evacuations ordered as a precaution, the agency said.

Video shot during the surge of water into the reservoir showed releases down the dam's concrete spillway as well as a torrent pouring over and through an emergency spillway on one side of the dam.

The California Department of Fish and Wildlife reported that all nine employees who work at the state-run hatchery immediately below the dam and live nearby had been evacuated, along with their families.

In a media conference, Tuolumne County Sheriff James Mele credited Hetch Hetchy personnel with acting quickly to alert local authorities and do what they could do release water from the suddenly swollen Moccasin Reservoir.

"For a few moments it was very stressful because we felt they were going to lose that dam," Mele said. "Even though it's a very small reservoir, the fact of the matter is it would have eroded a lot of the roadway" and disrupted travel for both local residents and those trying to reach Yosemite National Park.

Highways 120 and 49 were both closed as water cascaded down the dam's main concrete spillway and an emergency spillway. Highway 120 was later reopened; Highway 49 remained closed because water rampaging out of the dam's emergency spillway undercut parts of the road.

Don Pedro Reservoir, which can store a maximum of 2 million acre-feet of water, is owned by the Turlock Irrigation District. According to the Modesto Bee:

The Turlock Irrigation District said if Moccasin Dam does fail, it would have a very small impact on Don Pedro's storage capacity and the flows from Don Pedro.

TID spokesman Calvin Curtin said that is because Moccasin is such a small facility and it would raise the level of Don Pedro by less than one half of one foot. But the TID is looking at the potential impacts of debris, including logs, mud and rocks, that could enter Don Pedro if Moccasin failed and what steps it could take to mitigate that.