Moccasin Dam, used to regulate the Hetch Hetchy water system, is reported to be stable, shortly after the San Francisco Public Utilities Commission ordered an evacuation of all people “potentially in harm’s way.”
The dam and reservoir, on the Tuolumne River in California, experienced a major surge of water and debris after heavy rainfall on March 22. San Francisco PUC crews observed water leaking on the downstream face of Moccasin Dam, and the PUC activated its emergency action plan early on March 22. The evacuation, ordered in coordination with local officials, included people at the Moccasin Fish Hatchery and Moccasin Marina.
San Francisco PUC reports that the dam itself never overtopped and remains intact and that “at no time was there any threat to the Bay Area’s water supply.”
San Francisco PUC says it is started draining the Moccasin Reservoir into the much larger Don Pedro Reservoir just downstream. Additionally, crews “are actively inspecting the current state of the dam and the extent of any damage to it and its spillways.”
The Hetch Hetchy system originates from Hetch Hetchy Reservoir in California’s Yosemite National Park and provides water to San Francisco Bay area residents and businesses, as well as hydroelectric power for city services. More than 99% of all the electricity San Franciso PUC supplies comes from the Hetch Hetchy facilities. The system is composed of three hydroelectric powerhouses with a total combined capacity of nearly 400 MW. A 103.8-MW powerhouse is located at Moccasin Dam.