Tuolumne County reservoir flooded last year. After $22 million in fixes, it’s reopened

Repairs have finished at Moccasin Reservoir in Tuolumne County, about 15 months after storm runoff did damage there and at a nearby trout hatchery.

The City and County of San Francisco announced Monday that the nearly $22 million project was done. Moccasin is a small but crucial part of its Hetch Hetchy Water and Power System on the Tuolumne River.

The 554-acre-foot reservoir normally holds water discharged from a nearby hydropower plant and feeds it into a tunnel bound for about 2.7 million users in the Bay Area. On March 22, 2018, about 5 inches of rain fell over 24 hours and brought a torrent of water and debris into Moccasin Creek, upstream of the reservoir.

The operators used the two spillways to keep the flood from overtopping the main dam, barely, and water deliveries were never interrupted. But inspectors did find water seeping through the earthen dam, completed in 1930, as well as damage to one spillway, pipelines, culverts and other parts of the waterworks at Moccasin.

A bypass pipeline handled the water deliveries while the reservoir was repaired, said Dan Wade, director of water capital programs for the San Francisco Public Utilities Commission, by phone Monday. It manages the system for this city and about two dozen others in San Mateo, Santa Clara and Alameda counties.

“We’re pleased to have the reservoir back in operation because it increases the reliability of the system,” Wade said.

The flooding prompted the rescue of about a quarter of the 1.4 million trout at the Moccasin Creek Hatchery, operated by the California Department of Fish and Wildlife. It has resumed operation but will close for two months of further repairs on a summer date yet to be announced.

The hatchery raises five species of trout for planting in numerous streams and lakes. It is open to visitors from 7:30 a.m. to 3:30 p.m. daily.

The emergency at Moccasin prompted the evacuation of about 25 people on that March day. Wider impacts were avoided because Don Pedro Reservoir, which is 3,664 times as large, is just a mile downstream.