Seattle water by source

Seattle gets its water from two mountain rivers, and which one reaches you depends mostly on where you live. Both read as soft on the USGS scale.

Which one serves my home?

Seattle Public Utilities uses Green Lake as the rough dividing line, and says either source can serve other areas at times.

Cedar River

Homes south of Green Lake usually get Cedar River water. The Cedar supplied 62 percent of Seattle water in 2025. Source: SPU 2025 Drinking Water Quality Report.

• 1.55 grains per gallon for hardness, Cedar distribution sample, single sample, May 2026. Source: SPU Quarterly Surface Water Analysis (May 2026), non-regulatory data.

About 27 mg/L, which is our conversion of the published grains at 17.1 mg/L per grain. That reads as soft on the USGS scale.

South Fork Tolt River

Areas north of Green Lake usually get Tolt water. The Tolt supplied 38 percent of Seattle water in 2025. Source: SPU 2025 Drinking Water Quality Report.

• 1.53 grains per gallon for hardness, Tolt distribution sample, single sample, May 2026. Source: SPU Quarterly Surface Water Analysis (May 2026), non-regulatory data.

About 26 mg/L, which is our conversion of the published grains at 17.1 mg/L per grain. That reads as soft on the USGS scale.

Across the 2 Seattle areas above, the figures, with grains figures converted at our stated rate, run 26 to 27 mg/L, which sits in the USGS soft class. Each is a plant, well or area figure, not a reading at one address. (USGS hardness classes)

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Sources last checked October 27, 2026. Seattle Water Systems is not a water utility.