The falling ash is prompting the reservoir's managers to test the water quality two to three times a day, but water from the Hetch Hetchy Reservoir is drawn from the bottom of the reservoir, 270 feet below the surface.
"The more problem case we're looking at is the spring run off when the ash would come down from the water shed all at once when the snow melts," San Francisco Public Utilities Commission Deputy General Manager Michael Carlin said.
The SFPUC's general manager says the steps will be taken next spring if necessary.
The city of Santa Clara gets 12 percent of its water from Hetch Hetchy.
"We could run for an indefinite period on our other sources without an impact to customers," Santa Clara Water Utilities spokesperson Christopher de Groot said.
But in Palo Alto, 100 percent of the water comes from Hetch Hetchy, though a lot of the customers at the downtown University Café don't know that.
"I heard it from somebody last week that it comes from somewhere up north," Timo Rein said.
A spokeswoman for the city's utilities department says the SFPUC is reassuring the city.
"Basically we're being told that we don't have to worry at all about having a supply of water; San Francisco has several local reservoirs that have many months' worth of water in them," City of Palo Alto Utilities spokesperson Debra Katz said.
And if the ash from next spring's run off does degrade the water quality, the SFPUC is telling it's Hetchy Hetchy customers the water that is so pure now it doesn't need filtering will be filtered if need be.
"Which will certainly slow down the process of delivering it to us," Katz said.
The SFPUC reports the ash fall over the past 24 hours has been negligible. How bad it might get after next year's winter snows start melting depends to a great degree on how far this fire spreads into the Hetch Hetchy watershed.