State Water Board hears competing views on Tuolumne HRL after staff’s scientific‑basis supplement

The State Water Resources Control Board on Nov. 5 held a workshop to review a draft Scientific Basis Report (SBR) supplement that staff said will inform whether the Tuolumne River Healthy Rivers and Landscapes proposal (HRL) can be incorporated into the Bay‑Delta Water Quality Control Plan. The HRL, advanced by Modesto Irrigation District, Turlock Irrigation District and the San Francisco Public Utilities Commission (SFPUC), would combine schedule‑based flow commitments (base flows, a spring out‑migration pulse and a floodplain‑activation pulse) with non‑flow actions such as spawning gravel augmentation, constructed rearing habitat and predator management backed by roughly $80 million in local funding.

Staff summarized the SBR’s technical analyses — stock‑recruit modeling, two hydrology tools (the State Water Board’s monthly WSE model and a Tuolumne parties’ daily accounting spreadsheet), HEC‑5Q temperature modeling, and weighted‑usable‑area habitat estimates — and described how they compared the HRL against existing conditions and 30%, 40% and 50% unimpaired‑flow scenarios adopted in the board’s 2018 Bay‑Delta plan update. Staff said the SBR will be revised after public comment and will be submitted for independent scientific peer review in 2026.

Why this matters: board members and members of the public framed the discussion as a choice between two ways to pursue the board’s co‑equal goals — the 2018 unimpaired‑flow approach (30–50% unimpaired flow, with 40% specified as the February–June midpoint) and the HRL voluntary agreement. Supporters of the HRL emphasized it is locally funded and ready to implement, and they warned that a 40% unimpaired requirement would cause severe supplies and economic impacts in Bay Area retail systems that rely heavily on Hetch Hetchy/Tuolumne water. Opponents — environmental groups, independent scientists and many public commenters — said the SBR shows HRL produces only modest gains in habitat but does not guarantee the volumes, temperature improvements or Delta outflow protections necessary to meet the board’s biological goals.

Staff findings and technical debate

Staff’s stock‑recruit modeling identified spring water temperature as the strongest historical correlate with juvenile Chinook productivity; spring flow and suitable floodplain acreage were also positively correlated, though less strongly. Staff used the WSE model (monthly, 1922–2003) and a Tuolumne parties’ daily accounting spreadsheet (1999–2023) to represent operations. Both tools projected HRL increases in Jan–Jun flows during critical and dry years but diverged in wetter years because of modeling‑period and operational differences.

Temperature modeling indicated the HRL generally lowers modeled March–May temperatures relative to existing conditions, but benefits shrink in HRL drought off‑ramp years. Unimpaired‑flow scenarios (30–50%) generally produced cooler temperatures than the HRL during juvenile life stages, staff reported. Habitat modeling (weighted usable area, then temperature‑filtered) showed the HRL predicted increases in spawning and in‑channel rearing habitat versus existing conditions, but unimpaired scenarios produced more floodplain rearing area; none of the scenarios consistently produced the SBR’s highest floodplain thresholds at 100% of the doubling target, according to staff.

Positions from parties and public comment

Tuolumne parties and many elected officials and Bay Area water agencies urged the board to advance the HRL. They told the board the HRL is funded (~$80M self‑funded commitments), ready for near‑term implementation, and that operational flexibility and off‑ramps are necessary to preserve regional water reliability for millions of customers. The Tuolumne parties emphasized “more water, more habitat, more fish,” citing projects already in planning or early implementation and technical analyses projecting increases in juvenile production.

Conservation groups, independent scientists and fishing organizations said the SBR confirms that flow and temperature are the ‘‘master variables’’ for salmon viability and that the HRL, as proposed, does not provide sufficient, protected flows, adequate Delta outflow protections, or consistent temperature reductions to meet the board’s doubling goals. They urged the board either to implement the 2018 phase‑1 flow objectives or to require stronger, enforceable protections and clearer outflow guarantees before accepting HRL‑style voluntary agreements.

Next steps

Staff said public written comments on the draft SBR were due at noon on Nov. 7; staff will revise the SBR, submit it for independent peer review in 2026, and then the board will consider any proposed amendments to the Bay‑Delta plan to enable HRL implementation. The workshop produced detailed technical questions — including how and where HRL flows would be protected (compliance point at LaGrange is the HRL proposal), whether 1 or 2 pulses would be carried through the Delta, and how the HRL interacts with future water‑supply projects — that staff and parties said require further work as the board and agencies move from a supplement to possible plan amendments.

What the board will consider next

The SBR supplement is an early step in the board’s public process; it provides a comparative scientific record for the board and the public but does not itself change water rights or adopt policy. The major policy choices ahead are whether the board will move to amend the Bay‑Delta plan in a way that authorizes HRL‑type voluntary agreements, or instead proceed to implement the 2018 unimpaired‑flow objectives (30–50% unimpaired flows) as the board’s enforceable water‑quality standard. Staff and commenters emphasized that either path requires additional work on monitoring, accounting and the regulatory backstops (protecting flows beyond LaGrange, ensuring Delta outflow protections, and defining compliance and enforcement mechanisms).