Functional Flows Can Improve Environmental Water Management in California

By Ted Grantham, Jeanette Howard, Belize Lane, Rob Lusardi, Sam Sandoval-Solis, Eric Stein, Sarah Yarnell and Julie Zimmerman

Over the past three years, a team of scientists from universities, NGOs, and state agencies across California have been working to provide guidance on how to better manage river flows for freshwater ecosystems throughout the state. A key product of this effort is the California Environmental Flows Framework (Framework), a guidance document and set of tools to help managers and stakeholders develop environmental flow recommendations for California’s rivers. The technical approach of the Framework relies on the concept of functional flows, defined as aspects of a river’s flow that sustain the biological, chemical, and physical processes upon which native freshwater species depend. Under the functional flows approach, environmental flows would be maintained at levels that preserve key ecosystem functions—such as sediment movement, floodplain inundation, and environmental cues for species migration and reproduction—to satisfy native freshwater species needs. Flow recommendations developed through the Framework would address ecosystem water needs throughout the year and preserve the many benefits to people from healthy rivers and streams.

The Framework is a departure from previous environmental flow approaches in California. Here, we describe key attributes of the Framework’s functional flows approach and highlight how it relates to past approaches and those recently adopted by the State Water Resources Control Board (Water Board). As described in the Framework Guidance document, previous publications (Yarnell et al. 2015; Yarnell et al. 2020), and recent policy reports (Grantham et al. 2020), a functional flows approach would involve:

  • Managing for ecosystem functions: Environmental flow management in California has traditionally focused on habitat needs for individual fish species, such as threatened salmon populations listed under the federal Endangered Species Act. A functional flows approach would shift the focus from single species needs to ecosystem needs. By protecting the underlying functions that sustain river ecosystems, this approach is likely to deliver broad benefits for freshwater biota, including listed fish species, as well as support valued ecosystem services, such as clean water, fisheries, and recreation.
  • Mimicking natural flow variation: The traditional focus on single species (or even single life history stages of individual species) has also tended to favor static environmental flow requirements that vary little within seasons and across years. However, California’s native freshwater biota are adapted to the high natural variability in river flows. A functional flows approach preserves elements of natural flow variation upon which native species depend.
  • Coupling flows with physical habitat and water quality improvements: Environmental flow protections have traditionally focused on water quantity, but have often overlooked water quality (temperature, salinity, contaminant loads, and other parameters) and physical habitat (the form and composition of the river channel, banks, and floodplains) in supporting ecosystem health. A functional flows approach recognizes that ecosystem benefits of environmental water are realized only when flows are coupled with suitable water quality and suitable physical habitat.

The total quantity of water required to implement a functional flows approach in any given watershed can be expected to vary depending on its hydrologic setting and water year. However, it is likely that additional environmental water, beyond the volumes currently protected as minimum instream flows, will be needed in rivers subject to high human water demands, such as in the Sacramento-San Joaquin River Basin. This raises the question: how does the functional flows approach compare with recent environmental flow regulations adopted by the Water Board for this region?

In 2018, the State Water Resources Control Board adopted new flow objectives as part of the Bay Delta Water Control Plan for the San Joaquin Rivers and its major tributaries: the Tuolumne, Merced, and Stanislaus Rivers. Under the Plan, environmental flows for these rivers are expressed as a percent of unimpaired flows – a fixed proportion of expected natural flows unaltered by upstream diversions, storage, or imported water. For the months of February to June, the default flow objectives are 40% of unimpaired daily flow, based on a minimum 7-day running average, from each of the Stanislaus, Tuolumne, and Merced Rivers. The policy also sets minimum flow requirements for the San Joaquin River.

The Water Board policy has some similarities and differences with the functional flows approach described above. Like the functional flows approach, a percent-of-unimpaired flows approach is designed to preserve natural patterns of flow variation that support ecosystem health. Establishing flow recommendations as a fixed proportion of unimpaired flows ensures that seasonal, sub-seasonal, and interannual variation in flow are preserved. However, there is a risk that flows implemented as a percent-of-unimpaired flow may be insufficient to support some ecosystem functions targeted by the functional flows approach. This is because many stream functions have a threshold response and reducing flows by a fixed percentage cannot support all functions similarly. As stated by Poff et al. (1997), “clearly, half of the peak discharge will not move half of the sediment, half of a migration motivational flow will not motivate half of the fish, and half of an overbank flow will not inundate half of the floodplain.” To maintain ecosystem functions, it is likely more than 40% of unimpaired flow will be required at particular times of year, and for other times, potentially less.

Environmental water allocated as functional flows (red line) preserves aspects of natural flow variation needed to support ecosystem functions, but differs from the full natural flow regime (yellow) and fixed percent-of-unimpaired flow regime (blue). Modified from Grantham et al. 2020.

Despite this apparent limitation in the percent-of-unimpaired flows approach, provisions in the new policy would allow environmental flow recommendations to more closely follow a functional flows approach (Mount et al. 2020). Specifically, the policy allows for negotiated agreements in which unimpaired flows can vary over a larger range (30-50%) and potentially allows for even further deviation from these levels by treating environmental flows as a block of water. Conceptually, this block of water could be managed as functional flows by allocating it to meet functional flow targets at specific times of year (Grantham et al. 2020).

A budgeted water approach could also bring needed flexibility to the way environmental flows are managed in California (Mount et al. 2019). Environmental flows in California have historically been established through a variety of regulatory and legal processes, but tend to be prescriptive, requiring flows to be maintained at or above specific thresholds following a set flow schedule. This approach offers certainty to environmental interests and water users, but a fixed flow schedule makes it difficult to adapt to changing ecosystem conditions or take advantage of unique opportunities to enhance ecosystem benefits (Horne et al. 2018). For this reason, implementation of functional flows as an ecosystem water budget—a fixed volume of water for each year that can be flexibly managed to maximize ecosystem benefits and that could also be stored, traded, or leased—is a promising approach for improving the effectiveness of environmental flows in achieving ecosystem management objectives (Grantham et al. 2020).

The California Environmental Flows Framework is a holistic, science-based process to support resource managers, water agencies, and NGOs working to restore the health of California’s rivers. The Framework has been acknowledged by the Governor’s Office as a key element in building the resilience of California’s water system (State of California 2020). Environmental organizations and water user communities have both recognized the potential value of the functional flows approach advanced by the Framework (Grantham et al. 2020). The functional flows approach is also largely compatible with the 2018 Water Board policy, especially if implemented as water budgets supported by adaptable, robust governance. Additional work is needed to understand how the Framework will be used to inform the development of environmental flow programs and guide their successful implementation in diverse settings. Even so, the strong technical foundation, broad support, and new regulatory directives suggest the Framework’s functional flows approach has significant potential to improve the scale and effectiveness of environmental water management in the state.

Ted Grantham is a cooperative extension specialist at UC Berkeley, Jeanette Howard is the director of science for The Nature Conservancy’s water program, Belize Lane is an assistant professor at Utah State University, Rob Lusardi is a senior researcher at the UC Davis Center for Watershed Sciences, Sam Sandoval-Solis is an associate professor and cooperative extension specialist at UC Davis, Eric Stein is a principal scientist with the Southern California Coastal Water Research Project, Sarah Yarnell is a senior researcher at the UC Davis Center for Watershed Sciences, and Julie Zimmerman is the lead scientist for freshwater at the The Nature Conservancy. These individuals have been working with government agency staff to develop the California Environmental Flows Framework.

Further Reading

Grantham, T.E., Mount, J., Stein, E.D., and Yarnell, S.M. 2020. Making the Most of Water for the Environment: A Functional Flows Approach for California’s Rivers. Public Policy Institute of California Water Policy Center Report.

Mount, J., Grantham, T., Gray, B., and Hanak, E. 2020. Setting aside environmental water for the San Joaquin River. PPIC Water Policy Center Blog, 26 October 26 2020,

Horne, A.C., Kaur, S., Szemis, J.M., Costa, A.M., Nathan, R., Angus Webb, J., Stewardson, M.J. and Boland, N., 2018. Active management of environmental water to improve ecological outcomes. Journal of Water Resources Planning and Management 144: 04018079.

Obester, A., Yarnell, S., Grantham, T. 2020. Environmental flows in California,

Poff, N.L., Allan, J.D., Bain, M.B., Karr, J.R., Prestegaard, K.L., Richter, B.D., Sparks, R.E. and Stromberg, J.C., 1997. The natural flow regime. BioScience 47: 769-784.

State of California. 2020. Water Resilience Portfolio: In Response to the Executive Order N-10-19,

Yarnell, S., Obester, A., Grantham, T., Stein, E., Lane, B., Lusardi, R., Zimmerman, J., Howard, J., Sandoval-Solis, S., Henery, R., and Bray, E. 2018. Functional flows for developing ecological flow recommendations,

Yarnell, S.M., Petts, G.E., Schmidt, J.C., Whipple, A.A., Beller, E.E., Dahm, C.N., Goodwin, P. and Viers, J.H., 2015. Functional flows in modified riverscapes: hydrographs, habitats and opportunities. BioScience 65: 963-972.

Yarnell, S.M., Stein, E.D., Webb, J.A., Grantham, T.E., Lusardi, R.A., Zimmerman, J., Peek, R.A., Lane, B.A., Howard, J. and Sandoval‐Solis, S. 2020. A functional flows approach to selecting ecologically relevant flow metrics for environmental flow applications. River Research and Applications 36: 318-324

About jaylund

Professor of Civil and Environmental Engineering Director, Center for Watershed Sciences University of California - Davis
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6 Responses to Functional Flows Can Improve Environmental Water Management in California

  1. Gail Sredanovic says:

    Discussion of flows ultimately must include the historical role of beaver and benefits from their reintroduction. It is basic.

  2. Joseph Rizzi says:

    Salt Water Import is a KEY critical aspect in Flows – If we limit importing salty sea water to the ports of Sacramento and Stockton this would keep the Delta fresher with less flows required for aquatic life. Tidally controlled louvers can be place at many strategic places to reduce “Density Driven” Salt Water intrusion, with no more environmental harm than the channel dredging is causing the Salt Water intrusion today.

  3. A reminder that work pioneered by The Nature Conservancy beginning in 2004 on the Ecological Flows Tool and related substantial design and peer review workshops with over 70 agency and academic experts and managers who helped pioneer the functional flows approach with emphasis on overcoming sharp ecological (and water supply) trade-offs associated with different water operations. Most notably this work made a significant contribution to the theory behind managing and balancing flows for large numbers of competing uses (be they people, farms, cottonwood trees or delta smelt) by highlighting the largely unexplored power of “turn taking” in water management. See:

    Rather than trying to optimize conditions for all target species every year and having to pick the water winners and water losers, “turn taking” optimization (TTO) capitalizes on the natural ability of many aquatic and riparian species to withstand natural periods of unfavorable conditions. TTO creates flexibility and opportunities for different indicators to be successful in different years, informed by the frequency with which each species’ ecological needs should be met. As an individual EFT indicator is successful in a particular year, its priority in one or more subsequent years is reduced (and vice versa).

    The entire paradigm hinges on regulators and managers developing greater awareness of the value of flexibility to manage ecosystem trade-offs among multiple objectives — and providing approaches that enable this flexibility in a real-world setting. As noted in the SFEWS paper, “Flexibility begins with emphasizing holistic, integrated ecosystem-based management, monitoring the state of continuously fluctuating systems over relevant scales, and adjusting short term priorities accordingly. This, in turn, requires managers to adopt a mindset focused on adaptability and learning” rather than fixed rules.

    Worth checking out given the $1.x+ million investment by the State of California and partners over ~10 years!

    Other key references:

    and see other short summaries here:

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