Stanislaus County Water Report

Stanislaus County's drinking water received a D grade, scoring 42.9 out of 100. Over the past five years, EPA SDWIS records 131 health-based violations — a pattern that public water utilities are required to disclose and correct.

Under the Clean Water Act §303(d), EPA ATTAINS tracks whether waterways meet quality standards for drinking, recreation, and aquatic life (reporting cycle: 2022). A substantial 48.4% of assessed waterways are impaired (30 of 62 water bodies) across Stanislaus County's watersheds. The leading impairment causes are pathogens and toxicity. Impairment does not mean tap water is unsafe — it measures ambient waterway conditions upstream of treatment, not finished drinking water.

USGS NWIS gauge data (as of 2026-05-14T11:30:00.000-07:00) puts SAN JOAQUIN R at 576.0 cfs — well below its long-term average at 31% of mean — low-flow conditions worth noting for water-dependent ecosystems. Streamflow is a leading indicator of drought stress, sediment load, and dilution capacity: low flows concentrate pollutants and warm water temperatures, stressing aquatic life and, in surface-water-dependent systems, the source water quality for treatment plants.

EPA's Water Quality Portal (WQP) aggregates monitoring data from federal, state, and tribal agencies. Stanislaus County has extensive coverage with 68 active monitoring sites with 17,067 recent measurements on record. Predominant monitoring categories include organics, pesticide and organics, other. More monitoring sites generally indicate greater scientific attention to local water conditions — and provide the baseline data that regulators use to set future impairment listings.