San Luis Obispo County Water Report

San Luis Obispo County's water systems carry a failing grade, scoring 21.3 out of 100. Over the past five years, EPA SDWIS records 316 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 46.0% of assessed waterways are impaired (57 of 124 water bodies) across San Luis Obispo County's watersheds. The leading impairment causes are fecal coliform and dissolved oxygen. 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 2019-02-04T17:45:00.000-08:00) puts ESTRELLA R at 0.0 cfs — well below its long-term average at 0% 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. San Luis Obispo County has extensive coverage with 57 active monitoring sites with 15,446 recent measurements on record. Predominant monitoring categories include microbiological and physical. 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.