Los Angeles County Water Report

Los Angeles County earns a B grade for drinking water quality, scoring 66.5 out of 100. Over the past five years, EPA SDWIS records 268 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 51.6% of assessed waterways are impaired (148 of 287 water bodies) across Los Angeles County's watersheds. The leading impairment causes are pathogens and polychlorinated biphenyls (pcbs). 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 GABRIEL R AB WHITTIER NARROWS DAM CA at 9.9 cfs — well below its long-term average at 6% 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. Los Angeles County has extensive coverage with 223 active monitoring sites with 123,255 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.