Lewiston Water Hardness & Quality Report (2026)

Lewiston, Idaho, the Nez Perce County seat — a major north Idaho port city (Lewiston is the Nez Perce County seat and the only inland port in the Pacific Northwest accessible to oceangoing vessels — Lewiston is at the confluence of the Clearwater and Snake Rivers, and the Snake River has been dammed and dredged to allow commercial navigation all the way from the Pacific Ocean in Portland, Oregon; this makes Lewiston one of the most important agricultural shipping points in the inland Pacific Northwest), home of Lewis-Clark State College (a significant north Idaho regional college — named for the Lewis and Clark Expedition, which passed through the Lewiston area in 1805), a diverse Nez Perce County community with a significant Nez Perce tribal presence (the Nez Perce reservation is directly adjacent to Lewiston), and situated in the deep Snake River Canyon across the river from Clarkston, Washington — draws its municipal water supply from the Clearwater River via the City of Lewiston Water Division. Water hardness in Lewiston measures 132.5 mg/L — classified as moderately hard.

Lewiston's moderate hardness reflects the Clearwater River watershed's mixed basalt and granite geology. The Clearwater River at Lewiston–Nez Perce County drains: the Miocene Columbia River Basalt (calcareous-moderate); and the Precambrian Idaho Batholith granite (calcareous-poor). City of Lewiston treatment produces the moderate 132.5 mg/L.

At 132.5 mg/L, Lewiston residents encounter moderate scale accumulation. Monthly cleaning is recommended. City of Lewiston Water Division consistently delivers water meeting all Idaho DEQ and EPA Safe Drinking Water Act requirements.