Humboldt County is developing a Water Management Plan that will describe a range of proposed annual releases from Trinity Reservoir consistent with the 1959 water delivery contract between Humboldt County and the U.S. Bureau of Reclamation (Reclamation). The 1959 contract states that Reclamation shall release not less than an annual quantity of 50,000 acre-feet into the Trinity River for the beneficial use of Humboldt County and other downstream users (Contract Water).

The Water Management Plan will outline how Contract Water should be released for the benefit of fisheries in the Trinity River and lower Klamath River, with the primary goal of expanding a harvestable surplus of Tribal, recreational, and commercial fisheries. A set of annual Contract Water release scenarios were developed during five workshops conducted in 2022 and 2023 with interested parties including Humboldt County, state and federal resource agencies, tribal representatives, Reclamation, and the U.S. Department of the Interior Solicitor’s office. A suite of modeling and technical tools was used to analyze annual conditions with and without Contract Water releases.

This Modeling Report describes the modeling tools used to assess Contract Water release scenarios, including CalSim II, HEC-5Q, RBM10, sediment transport models, Chinook Salmon habitat models, and the Stream Salmonid Simulator. Results from all models are summarized to provide a comparative overview of modeled release scenarios to modeled baseline conditions.

Mean annual Contract Water release scenarios ranged from 50,000 acre-feet to 170,000 acre-feet, and varied in timing, magnitude, and duration, though all releases were made between October and April. As shown in Table ES-1, a key finding of this modeling report is Contract Water releases that had the greatest modeled increase in Chinook Salmon abundance relative to baseline conditions included those that released 50,000 acre-feet in the fall period from October through December as pulse flows or baseflows, and those that released 170,000 acre-feet from October through April as a combination of pulse flows and baseflows. Modeled beneficial effects on populations were primarily due to either (1) increases in habitat area during the spawning life stage in October through December, which decreased redd superimposition (e.g., the process of a later arriving spawner building a redd on top of an existing redd) and improved egg survival, or (2) increases in flow during the fry emergence and juvenile rearing life stage in March through April, which increased the fry and parr carrying capacity (e.g., the upper limit for the number of fry or parr that a habitat unit can support) of individual habitat units.

Another key finding of this report is all Contract Water scenarios that released at least 50,000 acre-feet annually from Trinity Reservoir had similar effects on Trinity Reservoir storage, Central Valley Project (CVP) storage, CVP contract water deliveries, and Sacramento River water temperatures. Whether these scenarios were released annually as a fall baseflow, fall pulse flow, spring pulse flow, or spring baseflow, they all resulted in similar storage patterns in Trinity Reservoir – an annual reduction in storage relative to the baseline that was relatively small in wetter years and larger in drier years. As a result of lower Trinity Storage levels, Trinity River Division (TRD) exports to the CVP were reduced. Because the timing of exports is similar each year, reaching a peak in July through September, the reduction to exports occurred at the same time each year, independent of Contract Water release timing, resulting in similar storage, CVP delivery, and water temperature effects in the Sacramento River basin portion of the CVP. The water temperature effects on the Sacramento River were limited to the months of July and August, relatively minor, and were primarily attributed to changes in storage, release magnitude, and release temperature from Lake Shasta, and not due explicitly to inflows from the TRD.