City of Sammamish Water Hardness & Quality Report (2026)

Sammamish, Washington, in King County on the Sammamish Plateau east of Lake Sammamish — one of the most affluent and fastest-growing cities in Washington State, a major tech-worker suburb east of Bellevue and Issaquah, known for its extensive trail system, Lake Sammamish State Park, and among the highest concentrations of technology company employees in the Pacific Northwest (Microsoft, Amazon, and numerous tech companies house substantial Sammamish workforces) — draws its municipal water supply from the Sammamish Plateau aquifer and Seattle Public Utilities Cedar River supply via the City of Sammamish Water Division. Water hardness in Sammamish measures 59.5 mg/L — classified as moderately soft.

Sammamish's moderate softness reflects the Sammamish Plateau's calcareous-poor Pacific Northwest glacial geology. The Sammamish Plateau aquifer draws from Pleistocene Vashon Stade glacial till and outwash — predominantly calcareous-poor Cascade Range volcanic and crystalline material. The Cedar River supply (from the Cedar–Tolt watersheds — Cascade Range volcanic and plutonic terrain, extremely calcium-poor) provides a very soft import supply to Sammamish. The blend of calcareous-poor Plateau aquifer and soft Cedar River import produces the moderate 59.5 mg/L.

With hardness at 59.5 mg/L, Sammamish residents enjoy moderately soft water. City of Sammamish Water Division consistently delivers water meeting all Washington DOE and EPA Safe Drinking Water Act requirements.