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.