Hetch Hetchy Filtration-Avoidance Watershed Protection (San Francisco, USA)

Since the O'Shaughnessy Dam was completed in 1923 and water deliveries to San Francisco began in 1934, the city's public utility (SFPUC) has drawn most of its drinking water from the Hetch Hetchy Reservoir, fed by snowmelt from a protected, high-elevation granite watershed inside Yosemite National Park. About 85% of SFPUC's water supply comes from this source. Because the water originates in a legally protected wilderness catchment largely free of grazing, logging and development, it meets the bacteriological and turbidity criteria the US EPA and California's drinking water regulator require for a "filtration avoidance" determination — one of only a handful granted to a major US city.

Rather than building and operating a filtration plant for this share of its supply, SFPUC maintains its exemption by funding ongoing watershed protection: security patrols, sanitary surveys carried out on a regular cycle, and a rolling multi-year watershed protection agreement with the National Park Service and US Forest Service covering the Tuolumne River watershed. The reservoir itself can store up to about 117 billion gallons, buffering supply through dry years.

The arrangement is not without risk. Because the exemption depends on the watershed's condition remaining pristine, degradation from wildfire, climate change or increased visitor pressure inside Yosemite could force SFPUC to filter this supply after all — a reminder that avoided-cost watershed protection is conditional, not permanent, and requires continuous monitoring investment rather than a one-off payment.

Read the full analysis: https://www.sfpuc.gov/accounts-services/water-quality/understanding-your-tap-water/water-treatment-process