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State Water Board adopts 2026 Integrated Report and 303(d) impaired waters list, adds new '5‑bio' subcategory for benthic effects
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Summary
The board unanimously adopted the 2026 Integrated Report/Section 303(d) list after staff refined benthic community assessment methods and added a new subcategory, 5‑bio, for waters with degraded benthic biology and at least one associated pollutant; staff will submit the list to US EPA by April 1, 2026.
On Tuesday the State Water Resources Control Board adopted the 2026 Integrated Report and the Clean Water Act Section 303(d) list, a routine but data‑intensive action that directs future water quality priorities. Staff delivered an extensive presentation on methods, data volumes (more than 1.4 million rows of submitted data, ~71,000 lines of evidence and over 15,000 assessments), and program improvements to enhance data transparency and automation.
A central topic was how the board evaluates benthic community effects (BCE)—a measure of stream biological integrity based on macroinvertebrate community condition using the California Stream Condition Index (CSCI). After public comment and technical review, staff added a new subcategory called "5‑bio" to identify water bodies that exhibit degraded benthic biology (CSCI scores below the tenth‑percentile reference threshold) and have at least one associated pollutant impairment for an aquatic life beneficial use. The staff clarified that a 5‑bio designation does not in itself require a TMDL for the degraded biology; TMDL development would follow only for associated pollutant impairments.
Staff and stakeholders debated the applicability of the statewide CSCI tenth‑percentile reference threshold (0.79) on Central Valley floor streams. In response to comments and pending analyses, staff moved Central Valley floor listings that relied on uncertain reference conditions into Category 3 (watch list) so they could be reexamined in an off‑cycle assessment with enhanced scientific review. Staff directed coordinated research with the SWAMP Bioassessment Program and the Bioassessment Working Group to refine thresholds and definitions for specific water‑body types and ecoregions.
Public commenters—including environmental, utility and regional organizations—praised staff’s work and urged additional clarity about how 303(d) listings are used in permitting. Staff said the division completed a review of how 303(d) listings interact with permitting programs and will continue coordination with regional permitting staff to ensure appropriate use of listing information in drafting permit requirements. The board adopted the resolution by unanimous roll call and staff will submit the adopted list to U.S. EPA by April 1, 2026.
