The fire burning around San Francisco's freshwater supply in Yosemite National Park may create water quality problems for The City — but years down the road, after it rains.
On Monday, the Rim Fire reached the Hetch Hetchy Reservoir — which contains about 85 percent of the Tuolumne River water that's piped 167 miles to the taps of 2.6 million Bay Area customers. And by midday Wednesday it had burned along about a four-mile stretch of the reservoir's southern edge, according to Cal Fire and the U.S. Forest Service.
Fueled by dry conditions, the fire is burning away trees and vegetation, without which the land around the reservoir becomes more susceptible to erosion during even a small rainstorm.
"Rainfall rates as low as two-fifths of an inch in an hour have triggered slides" in other parts of the state, according to Dr. Laurel Larsen, a professor of geography at UC Berkeley.
The worst slides would not be expected this winter but the next, after burned trees die and the root systems that hold hillsides together decay, said Dr. Leonhard Blesius, a professor of geography and environment at San Francisco State University.
"Certainly there will be erosion, which will send sediment into the rivers," he said. By 2016 and thereafter, trees and grasses should grow back to give the hillsides their original strength.
The City's current water quality is the same as it was before the fire, according to the San Francisco Public Utilities Commission, which has nonetheless piped several months worth of water supply away from the fire.
The natural sediment that appears certain to fall into the reservoir is nontoxic, but enough dirt could make The City's drinking water cloudy.
That would require the SFPUC to temporarily shut down the reservoir until conditions settle, or to begin filtering the Hetch Hetchy supply, which is remote enough to be one of the few municipal water supplies in the country that does not require filtration.
The SFPUC's newly refurbished treatment plant in Sunol has the capability to filter 150 million of the 270 million gallons of water delivered a day, SFPUC spokesman Tyrone Jue said.
The $4.6 billion voter-approved Water System Improvement Project connected The City's water supply to other utilities, from which the SFPUC could buy fresher water while a cloudy Hetch Hetchy settles down.
Hetch Hetchy is already shut down several times a year, Jue said.
"A lot of times, we let the turbidity settle," Jue said. "It's just dirt — we let gravity do its job."
Remote and rugged conditions mean Cal Fire and the Forest Service are relying on aircraft to fight the flames.
To keep the water clean, the SFPUC has requested that the aircraft do not draw water from Hetch Hetchy, and do not drop fire retardant — which is a "biodegradable" mix of phosphorus and nitrogen deemed safe for use on wildlife — in the watershed.
There was a drop Wednesday of fire retardant in the general vicinity of Hetch Hetchy, but it was confirmed to not be within the watershed, Jue said.