Sammamish Water Hardness & Quality Report (2026)

Sammamish, Washington, in King County on the Sammamish Plateau east of Lake Sammamish — one of the fastest-growing cities in Washington State, an affluent tech-community suburb (Microsoft, Amazon, and Boeing employee corridor), a young family community with some of the highest median household incomes in Washington — draws its municipal water supply from the Cedar River and Tolt River watersheds via Seattle Public Utilities (SPU), distributed to Sammamish by the City of Sammamish Water Division. Water hardness in Sammamish measures 60 mg/L — classified as moderately soft.

Sammamish's moderate softness — slightly harder than Seattle (15 mg/L) and Shoreline (15 mg/L) on the same SPU Cascade supply — reflects the Tolt River watershed blending in the Sammamish Plateau distribution zone. The Tolt River watershed (draining the Snoqualmie Pass area — Snoqualmie Batholith granite and Cascade Range volcanic terrain) is slightly harder than the pure Cedar River supply due to minor calcareous formation contact in the Tolt watershed. The SPU blended supply distributed to the Sammamish Plateau produces the moderate 60 mg/L — still among the softest in the Puget Sound region.

With hardness at 60 mg/L, Sammamish residents enjoy moderately soft water with minimal scale challenges. City of Sammamish Water Division consistently delivers water meeting all Washington DOE and EPA Safe Drinking Water Act requirements.