The third-largest wildfire in California history — which raged for more than two months in and around San Francisco’s source of drinking water, the Tuolumne River — had no ill effects on The City’s water supply, according to the results of a survey delivered Tuesday to the San Francisco Public Utilities Commission.
The fire burned more than 257,000 acres of national park and forest, including 8 square miles’ worth of land in the Hetch Hetchy watershed. However, much of that was not burned severely, including the surroundings of Hetch Hetchy Reservoir in Yosemite National Park.
Repeated tests of water collected from the reservoir and farther downstream detected no change in quality.
The biggest risk to water quality was soil, rock and other sediment loosened by burned vegetation sliding into the reservoir during winter rains.
Water quality historically has been affected only by “major storm events,” according to the SFPUC.
And though a dry winter with little to no rain in the Sierra Nevada mountains may spell water trouble down the road, the lack of precipitation has kept clean water flowing to the Bay Area, according to the SFPUC.
There have been two fires — one in 1948 and another in 1999 — that burned more of the watershed than the Rim Fire.