Short definition
A surface water source is a river, lake, or reservoir that feeds a public water system. Most of Washington’s largest municipal utilities use surface water from upland Cascade watersheds: Seattle’s Cedar and Tolt Rivers, Tacoma’s Green River, Everett’s Spada Lake / Sultan River. Surface water is typically softer than groundwater but requires substantial treatment because it’s more exposed to runoff, sediment, and pathogens.
What it is
Surface water comes from the visible water in the landscape — streams, rivers, natural lakes, man-made reservoirs. A municipal surface-water system collects from its watershed, runs the water through treatment (coagulation, sedimentation, filtration where required, disinfection), and pumps it through distribution mains to customers.
The chemistry profile of upland Pacific Northwest surface water is distinctive: low total dissolved solids, low calcium, low magnesium, slightly acidic before treatment. That makes the water “soft” — no scale problems on water heaters or fixtures — but it’s also the chemistry that historically caused copper pinhole leaks and blue-water staining in older Seattle and Tacoma homes before utilities upgraded their corrosion-control treatment.
Why it matters to a homeowner
If you live in Seattle, Tacoma, Bellevue (Eastside on Seattle wholesale), Everett, or most of the I-5 corridor, you’re on surface water. Practical implications:
- Soft water means no scale — water heaters, faucets, and fixtures don’t accumulate the white crusty buildup typical of hard-water regions. Annual heater flushing is less critical than in Spokane.
- Soft, slightly acidic legacy water is the chemistry behind blue-green stains in tubs, sinks, and toilet bowls, and behind copper pinhole leaks in pre-1990 homes. Modern corrosion-control treatment has largely fixed this going forward, but the scarred copper remains in older stock.
- Different treatment chemistry by city. Seattle adjusts pH with lime and CO₂; Tacoma uses caustic soda; both achieve similar corrosion-protective pH. The end-user difference is minimal.
- Both Seattle and Tacoma use free chlorine, not chloramine, which matters for aquariums, certain rubber gaskets, and standard carbon-filter behavior.
Washington note
The three major Washington surface water systems:
- Seattle Public Utilities — Cedar River watershed (south of Green Lake) and South Fork Tolt River watershed (north). Both protected upland Cascade catchments. Cedar operates under an EPA unfiltered-surface-water variance with ozone disinfection added in 2004; Tolt has been filtered since 2001 with ozone added in 2004. Service area: City of Seattle plus wholesale customers across King County (~1.5 million people regionally).
- Tacoma Water — Green River watershed (~95% of supply), 231 square miles in the Cascade foothills. Filtered since 2015 (Green River Filtration Facility); supplemented by groundwater wells on the North Fork Green River and inside Tacoma city limits.
- Everett — Spada Lake / Sultan River system, serving Snohomish County’s largest cities.
These systems are regulated under WAC 246-290 (Group A) by the Washington Department of Health Office of Drinking Water.
For a more detailed look at the specific city profiles, see the Seattle tap water and Tacoma tap water entries.