According to this article, authorities are rapidly trying to fill up reservoirs away from the wildfire region with water from Hetch Hetchy before it becomes more contaminated by ash from the Rim Fire. The intake valves are down about 300 feet from the surface, and ash has not really reached that level yet.

Not an immediate threat of impatcting the city's water supply in any way since these outlying reservoirs already store many months of water for San Francisco. At some point, the water coming from Hetch Hetchy might require some additional filtering for ash and later on, from post-fire sediment.