The blaze, which started Aug. 17 and has charred more than 187,000 acres, reached the area around the reservoir this week, SFPUC spokesman Tyrone Jue said.
The water, which supplies 2.6 million customers in San Francisco and elsewhere in the Bay Area, has maintained the same turbidity, or cloudiness, it had before the fire, according to the commission.
The fire is burning on the south side of the reservoir, and Jue said the rocky terrain around Hetch Hetchy is preventing adverse impacts.
"There's not much vegetation around the perimeter of the reservoir," he said. "It's basically in a giant granite bowl."
Some ash has fallen on the surface of the reservoir, but the water sent to San Francisco is withdrawn from 260 feet below the surface, according to the commission.
Jue said that although the drinking water supply has not been affected so far, the SFPUC could have to deal with erosion problems if the flames reach deeper into the Hetch Hetchy watershed. Winter or spring runoff could affect water quality, he said.
"That's where you might have more of an issue," Jue said.
He said if a problem arises, the SFPUC will "make the appropriate adjustments to the system."
The commission recently began increasing the amount of water delivered to the Bay Area to maximize the supply stored locally, and also has agreements with other nearby utilities if supplements are needed.
SFPUC crews are also working to repair hydroelectric facilities that were damaged by the fire last week. All of San Francisco's municipal electric customers continue to be fully supplied, according to the commission.
The Rim Fire had burned 187,466 acres as of this morning and was only 23 percent contained, according to the U.S. Forest Service. The cause of the fire remains under investigation.