Managing the delicate balance between reliable water delivery and environmental stewardship in California requires rigorous scientific backing. At the request of the US Bureau of Reclamation (USBR), the National Academies of Sciences, Engineering, and Medicine (NASEM) recently conducted a comprehensive review of the scientific activities supporting the long-term operations of the Central Valley Project (CVP) and State Water Project (SWP). An 18-member committee led this work, and the report underwent independent peer review before its release. Issued in November 2025, this report serves as the inaugural product in a planned series of biennial reviews designed to solve complex problems and inform public policy.
The committee’s charge was specific: assess the science informing three high-stakes actions managed by the USBR and the California Department of Water Resources (CDWR). These actions are central to daily operations and are critical for both water customers and the protection of six species listed under the Endangered Species Act. Because of their impact, these areas remain subjects of significant controversy.
The report focuses on the science used to inform the following:
- Old and Middle River flow management
- Shasta cold water pool management
- Summer-fall Delta smelt habitat action
In addition to assessing the current state of science, the committee provided recommendations on how to improve monitoring, modeling, and decision-support tools. The goal is to accurately assess the impacts of the projects on the operations listed above.
At a November webinar coinciding with the report’s release, Committee Chair Dr Peter Goodwin, Emeritus from the University of Maryland Center for Environmental Sciences, presented the report’s findings and recommendations.
The Delta is a very large and complex system, and the three focal management actions in this report are particular examples of the broader water and environmental problems in California and the western United States. “These problems are all sizable and involve both near-term and long-term conflicts and uncertainties,” said Dr. Goodwin. “Understanding these problems and the pursuit of effective solutions benefits from long-term scientific programs and communication activities. Relevant peer-reviewed science does not happen overnight, and this long-term commitment and perspective are needed.”
SHASTA COLD WATER POOL MANAGEMENT
Shasta Dam forms the largest reservoir within the Central Valley Project, with a storage capacity of 4.5 million acre-feet. However, the dam has significantly impacted winter-run Chinook salmon, as its construction has blocked access to their historical spawning grounds in upstream headwaters. Currently, the salmon are restricted to spawning in a limited stretch of river immediately downstream of Keswick Dam, the furthest point they can migrate.
The Shasta Coldwater Pool Management Action is designed to maintain sufficiently cold water in the reaches downstream of Keswick Dam during summer to support adult winter-run Chinook holding, spawning, and egg incubation. To accomplish this, the U.S. Bureau of Reclamation (USBR) conserves cold water in Shasta Reservoir until summer. It then utilizes a Temperature Control Device (TCD) to blend water from different temperature layers within the reservoir as needed.
Despite these efforts, the area with suitable water temperatures for spawning and egg survival has continued to shrink over the last 25 years. Improving habitat conditions in this critical reach is necessary for the species’ survival. The USBR’s cold-water management strategy is informed by comprehensive monitoring and a new water-temperature modeling platform developed by Reclamation.
“This level of sophistication is needed as managing the cold water pool is highly complex,” said Dr. Goodwin. “It is not simply the volume of runoff in a given year, but there are a host of other compounding factors that must be understood and factored into decisions, not least the volume and temperature structure of the cold water pool and the contribution of upstream and downstream tributaries.”
Key recommendations include:
The current three-pronged approach should be embraced. These are improving conditions downstream of Keswick Dam, continued supplementing through hatcheries, and the reintroduction of salmon to their natal spawning areas upstream of Shasta and in Battle Creek.
Enhanced monitoring of Shasta Reservoir and relevant rivers could improve management. There are specific locations where additional temperature sensors set into the river could enhance the water temperature modeling platform’s predictive capacity, and additional monitoring in the reservoir that could give insights into the vertical mixing processes that influence the evolution of the cold water pool throughout the year, as well as the temperatures of water released through the temperature control device to the Sacramento River. Reclamation and UC Davis are working to gather more information on the levels from which water is drawn into the temperature control device, which would allow dam operators to have greater certainty and control over the temperature of released water.
The high levels of unattributed mortality among winter-run Chinook salmon eggs must be better understood to develop management actions that increase confidence in using egg-to-fry survival as the principal focus for cold-water pool management. Ongoing studies show promise, and investigating temperature, dissolved oxygen, and flow-through rates could evaluate the benefits of channel and floodplain restoration, gravel augmentation, and pulse flows in sustaining pool-riffle sequences and other features conducive to spawning.
OLD AND MIDDLE RIVER
The Delta consists of a complex network of tidal channels with variable flow dynamics. Pumping operations conducted by the Central Valley Project (CVP) and State Water Project (SWP) in the south Delta pull water from the Sacramento and San Joaquin rivers toward the south Delta. These activities alter established flow paths and contribute to issues such as fish entrainment.
Under natural conditions, these channels experience both downstream (positive) and reverse (negative) flows associated with ebb and flood tides, resulting in an overall net positive daily flow. However, water exports by the CVP and SWP can result in a net negative daily flow, or reverse flow, in portions of these channels.
Old and Middle River (OMR) flow management is designed to regulate freshwater exports at the Jones and Banks pumping plants, typically from December through June. This period aligns with the presence of sensitive life stages of listed fish species in the Delta. The primary objective of OMR flow management is to fulfill water delivery requirements while reducing adverse impacts on protected fish species resulting from export operations.
To meet the requirements outlined in biological opinions by the U.S. Fish and Wildlife Service (USFWS) and National Marine Fisheries Service (NMFS), the U.S. Bureau of Reclamation (USBR) and California Department of Water Resources (CDWR) manage the average daily flows in the Old and Middle River (OMR) corridor, a process known as OMR flow management.
The committee was tasked with evaluating the current scientific understanding of OMR flow management. The scope of this evaluation included recommending enhancements to modeling techniques, monitoring protocols, and decision-support tools to more effectively assess the impacts of OMR flow management.
The committee’s assessment found that the existing OMR flow management strategy, which uses fish takes at the pumps, flow levels, and turbidity as triggers for pumping limitations, has a reasonable scientific basis. However, significant uncertainties exist regarding how well each trigger correlates with fish protection.
Key recommendations include:
The underlying scientific basis for OMR’s thresholds and corresponding export reductions should be made available for appropriate review.
To move OMR flow management beyond the use of salvage as a proxy for impacts on fish in the Delta, monitoring and modeling in support of the action should evolve to address:
* What is the zone (or sphere) of influence of operations?
* What proportion of any given fish population is under that influence?
* What is the impact on fish in these regions?
* What are the consequences of these impacts on the full population?
Expanded modeling efforts should build on USBR’s zone of influence analysis to study changes in temperature, submersed aquatic vegetation, salinity, and other water quality parameters across regions under a variety of Delta flows, spanning dry to wet years and at a range of pumping rates.
SUMMER-FALL HABITAT ACTION
The third action examined by the Committee addresses Summer Fall Habitat. The continued decline of the Delta smelt is partly linked to insufficient habitat and food availability during the summer and fall months. Due to critically low population numbers, detecting changes in abundance remains difficult.
Ecological disruptions in the Delta and Central Valley have been extreme, particularly in summer and early fall. Consequently, management efforts focused on enhancing habitat by manipulating flows to influence salinity and food availability are considered scientifically reasonable.
The SFHA seeks to improve Delta smelt fitness—specifically growth, survival, and recruitment—by mitigating human-driven habitat modifications. The strategy focuses on aligning smelt distribution with preferred low-salinity habitat in Suisun Bay and Suisun Marsh. This approach targets a specific seasonal bottleneck affecting juvenile and subadult life stages.
The SFHA consists of three primary elements: Fall X2 management, operation of the Suisun Marsh salinity control gates, and supplemental water releases. Fall X2 management involves increasing freshwater releases to push “X2″—the location where bottom salinity averages two parts per 1,000, measured from the Golden Gate Bridge—further downstream, thereby expanding the area of low-salinity habitat in Suisun Bay and Suisun Marsh. The operation of the Suisun Marsh gates is designed to actively modify marsh salinity and enhance food provision for Delta smelt. In addition, the plan allows for potential supplemental water releases of up to 100,000 acre-feet (TAF) to further support habitat objectives.
The SFHA is controversial due to the high cost of water released into the estuary, which could otherwise be exported or stored. With agricultural water prices south of the Delta estimated at $300 per acre-foot, a reduction in exports of 100 TAF per year would result in an approximate economic loss of $30 million annually.
The Committee commends the Bureau of Reclamation and the Department of Water Resources for adopting an ecosystem approach to this complex issue. However, several challenges remain:
Attribution of results remains challenging, as the system’s complexity and confounding factors make it difficult to clearly isolate the SFHA’s specific impacts from broader system dynamics. Scientific uncertainty also persists, particularly regarding the relationships between the action, food availability, and Delta smelt recruitment. In addition, extremely low population numbers pose considerable challenges to monitoring efforts and raise questions about the potential role of experimental releases of cultured fish.
Key recommendations include:
Agencies are encouraged to pursue the identified special studies related to food and to commit to implementing the Suisun Marsh Salinity Control Gate component of the SFHA over multiple years in sequence.
Development of a process-based Delta smelt model within a spatial food-web framework could substantially enhance the implementation of the SFHA and improve understanding of the effectiveness of its individual components. Such a model would help address key management questions about the magnitude, timing, frequency, and location of flow actions, as well as their ecological rationale. However, establishing this modeling capacity will require targeted data collection, dedicated experimentation, and likely multi-year collaboration and investment across multiple agencies.
An annual decision process for the SFHA, founded on a series of environmental triggers that encompass more than water year type, should be considered. The lessons from previous summer fall habitat actions, such as in 2017, indicate that the success of fall x2 in a wet or above-normal year may also depend on conditions in the previous year. Temperature effects or other processes, a set of triggers, for example, related to temperature and food resources for the Suisun Marsh salinity control gate, could be established based on expectations developed early in the year, which would identify what conditions the action would proceed and how it would be implemented.
OVERARCHING RECOMMENDATIONS
The researchers identified broad issues that spanned across all three actions, leading to recommendations including:
A formal feasibility study should assess the potential for a Science Hub for the Bay-Delta and its watershed. There are opportunities to enhance collaborative science and accelerate the pace and effectiveness of Bay Delta watershed science endeavors, possibly through an interagency science hub, also known as a collaboratory. The report points to several examples of this being achieved in the Bay Delta watershed and elsewhere. “The purpose would be to serve interagency needs without diminishing the role or resources of cognizant agencies, but it could include interagency approaches to the problems, monitoring tools, as well as providing cross-agency training in the use of these technologies and models,” said Dr. Goodwin. “And most importantly, it should be designed to find solutions to problems that are very difficult to address with an existing agency or academic silos.”
Finer-scale models are needed for water and ecosystem management in the Delta and its watershed. CalSim, now widely used to evaluate ecosystem impacts, would benefit from a finer time step (perhaps one week).
DWR and USBR should coordinate more closely and develop shared standards and protocols for modeling climate change impacts on the CVP and SWP, especially given increasing evidence of more frequent compound events (ie, multiple extreme events occurring simultaneously) and cascading events (ie, extremes occurring in sequence). Department of Water Resources and Reclamation are both considering the impacts of a changing climate – variable hydrology, greater variability from year to year related to the Central Valley Project and the State Water Project. Much could be learned from closer collaboration and sharing standards and protocols with the different but complementary approaches.
In conclusion …
“In summary, the three actions explored in this study are necessary, but alone are insufficient to recover the target and endangered species,” said Dr. Goodwin. “However, the committee felt there is cause for optimism if these actions are modified and conducted in conjunction with other actions, and the recommendations of taking an ecosystem approach, expanding what’s already being done in this sphere, will allow these actions to be better coordinated. And the objective throughout this report was to be proactive, positioning the available science for improved science, informed discussions, policy making, and management decisions.”
