Seattle's Cedar River Municipal Watershed — Filtration-Avoidance Water Source Protection (USA)

Since the early 1900s the City of Seattle has owned and managed almost the entire Cedar River watershed - roughly 90,000-100,000 acres of forested land in the Cascade foothills - as a closed, undeveloped source-water catchment with no agriculture, industry, logging or public residential use permitted inside it.

The programme protects source-water quality well enough to avoid building a conventional filtration plant, while meeting Endangered Species Act obligations to salmon and trout species in the watershed.

Seattle Public Utilities (SPU) manages the closed catchment and disinfects supply with ultraviolet light and ozone instead of filtration. In April 2000 the City signed a 50-year Cedar River Watershed Habitat Conservation Plan (HCP) with the US Fish & Wildlife Service and NOAA Fisheries, an Endangered Species Act incidental-take permit covering Chinook, coho and sockeye salmon, steelhead and coastal cutthroat trout. The watershed was designated a no-logging forest reserve under the HCP, and in 2003 the City removed the century-old Landsburg Diversion Dam. SPU staff patrol 50-80 miles of watershed roads daily.

In 2004 the EPA granted Seattle's source-protection programme a 'Limited Alternative to Filtration' (LAF) determination, one of only a handful ever issued to a large US water system. SPU estimates the avoided filtration-plant construction cost at more than $100 million. The unfiltered Cedar supply provides roughly 70% of drinking water to about 1.4 million people in the greater Seattle area. The 2003 Landsburg Dam removal reopened about 17 miles of river habitat to migrating fish, and the City retains ownership of about 99.8% of the hydrographic basin.

As with New York City's similar Catskills programme, filtration avoidance is a conditional, periodically reviewed EPA determination rather than a permanent guarantee. A 2015 drought forced SPU to invest in deeper reservoir pumping infrastructure, and rising wildfire and drought risk in the Pacific Northwest could eventually threaten the 'no filtration needed' status that has held since 2004.