I've been reading about Hetch Hetchy drinking water and was surprised to learn that it's not filtered.

From Wikipedia:

Water from Hetch Hetchy is some of the cleanest municipal water in the United States; San Francisco is one of six U.S. cities not required by law to filter its tap water, although the water is disinfected by ozonation and, since 2011, exposure to UV. The water quality is high because of the unique geology of the upper Tuolumne River drainage basin, which consists mostly of bare granite; as a result, the rivers feeding Hetch Hetchy Reservoir have extremely low loads of sediments and nutrients. The watershed is also strictly protected, so swimming and boating are prohibited at the reservoir (although fishing is permitted at the reservoir and in the rivers which feed it), a measure which is considered unusual for US lakes outside the region.


The surface water in the Hetch Hetchy Reservoir is treated, but not filtered because it is of such high quality.

From UC Davis:

The only difference between the water sitting in the afterbay at Moccasin Power House and the water coming out of the taps in San Francisco is a tablet of chlorine (fluoride and a couple other chemicals are added as well). I laughed when I saw the ducks and swallows leaving their droppings in the water and how all the unsuspecting San Franciscans will be filling their cups with the same water.

I think I understand that the water coming into the Hetch Hetchy Reservoir is clean, but that clean water still has fish and bugs in it. I've been trying to read more about this, but most articles just say it's not filtered. So at the risk of asking a silly question: why doesn't Hetch Hetchy drinking water ever have debris in it? If it's not filtered at all, shouldn't I see the occasional dead bug?

  • $\begingroup$ Given the info you provide, one might well assume nature is doing the filtering. Or are you missing the other things that get added like flouride etc ? $\endgroup$
    – Solar Mike
    Commented Apr 18, 2022 at 3:53
  • $\begingroup$ @SolarMike I'm wondering how nature filters out things like bugs, dirt, fish, etc. $\endgroup$ Commented Apr 20, 2022 at 16:29

2 Answers 2


The water from Hetch Hetchy Reservoir is disinfected with chloramine (chlorine and ammonia) and UV light.

San Francisco also gets water from other reservoirs and sometimes moves water from Hetch Hetchy to other reservoirs. This water is filtered - on average about 30% of the total supply.

Every few years when they do maintenance on the conveyance system, all system water is filtered for a month or two.

As far a dead bugs, they’re probably at the top of the water column and water is withdrawn at the bottom. Water from Hetch Hetchy also generates hydropower twice before being moved across the valley so any bugs would be quite pulverized by the pressure.

Finally, while the engineering is impressive, the reservoir destroyed an environmental treasure. The reservoir should be relocated and the valley, comparable to Yosemite Valley, should be restored. See hetchhetchy.org

  • $\begingroup$ Wouldn't other debris come from the bottom, like dirt? $\endgroup$ Commented Apr 20, 2022 at 16:30

To make one minor addition to Spreck's excellent answer (hi Spreck!) the Hetch Hetchy water flows slowly through the (artificial) Crystal Springs reservoir before entering SFO. When it enters the reservoir, it is naturally nutrient-deprived (minimizing the chances of, for example, an algal bloom in the reservoir) and has ample time to drop any (small) amounts of suspended sediment into the bottom of the reservoir system.


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