State Water Board workshop finds Tuolumne voluntary agreement adds modest flows; debate remains over unimpaired‑flow standard

The State Water Resources Control Board on Nov. 5 held a public workshop to review a draft scientific‑basis report supplement (SBR) assessing the Tuolumne River Healthy Rivers and Landscapes proposal (HRL), a voluntary‑agreement package developed by the Modesto Irrigation District, Turlock Irrigation District and the San Francisco Public Utilities Commission.

State Water Board staff summarized modeling and monitoring analyses that will inform whether to amend the Bay‑Delta Water Quality Control Plan. The report compares: existing (2023) operations; the HRL alternative (schedule‑based base flows plus spring and floodplain pulses and specific habitat commitments); and 30%, 40% and 50% of unimpaired‑flow alternatives adopted as a framework in the 2018 Bay‑Delta Plan.

The SBR finds the HRL would add modest new water in many dry and critical years — staff presented a modeled range of roughly 2,000–12,000 acre‑feet of ‘‘new’’ Tuolumne flow in many years and roughly 7,000–17,000 acre‑feet of additional modeled Delta outflow under some assumptions — and that the HRL’s non‑flow actions (gravel augmentation, constructed rearing habitat, large woody debris, predator management and early habitat projects) would likely increase temperature‑filtered spawning habitat by about 14%, in‑channel rearing by about 8% and modeled floodplain habitat by about 17% compared with existing conditions. The SBR also shows large uncertainties in those estimates and stresses that a robust monitoring and adaptive management program would be needed to verify outcomes.

Staff emphasized that many of the SBR’s most powerful statistical predictors of juvenile Chinook productivity are spring water temperature, spring flow magnitude and meaningful floodplain inundation. The report’s…