By Sarah Yarnell, Ethan Baruch, Andrew L. Rypel, and Rob Lusardi

As California grapples with ongoing water management challenges, the question of “how much water to leave in streams” to support native fishes and aquatic species often arises. There is no easy answer to this question, as evidenced by the more than 200 environmental flow assessment methods that exist globally. In California, there has been movement towards using a Functional Flows approach, as described in the California Environmental Flows Framework (CEFF) to answer this question more scientifically. Several previous blogs have covered CEFF and Functional Flows (see further reading below), and the California Environmental Flow Technical Working Group meets quarterly to discuss and share related research, case studies, and applications. For people learning about this work, two common questions are often asked: “Has a Functional Flows approach been implemented anywhere in the state?” and “Is there empirical evidence that a Functional Flows approach really supports California native fishes?”  In short, the answers are “to a certain extent” and “yes”.  A recent study by Baruch et al. in 2024 in Ecological Applications helps explain these answers. 

Putah Creek in northern California originates in the headwaters of California’s Coastal Range and drains to the Sacramento River, providing habitat to various native resident and anadromous fishes below Monticello Dam and Putah Creek diversion dam, a little further downstream. Following a multiyear drought in the early 1990s that caused extensive drying and fish kills in lower Putah Creek, a lawsuit was filed to augment flows below the diversion dam (Putah Creek Council v. Solano Irrigation District and Solano County Water Agency, Sacramento County Superior Court No. 515766). The court-ratified Putah Creek Accord (Accord hereafter) was subsequently implemented in 2000, with instream flow requirements that were intended to benefit native fish. 

Thanks to consistent funding and commitments from Solano County Water Agency, monitoring of fish populations occurs annually since the mid-1990s, resulting in one of the largest long-term fish assemblage datasets in California. As a result, writings on the history, ecology, and management of Putah Creek are frequent topics on this blog (see further reading below). In general, long-term data show that following implementation of the Accord, the fish assemblage shifted drastically from dominance by non-native to native species in the 20 km below the diversion dam. In the same reach, native fish abundance increased, on average, from 53% to 93% by 2019. But what specifically about the Accord flows promoted native fish recovery?  

We examined the response of Putah Creek’s fish assemblage to changes in aspects of the streamflow regime known to support ecosystem functions – in this case, changes in functional flow components as described in CEFF (e.g., fall pulse flow, wet-season baseflow, spring recession flow, and dry season baseflow). Although Accord flows were not strictly designed following a functional flows approach, having occurred almost two decades earlier, they explicitly included aspects of a natural flow regime known to support native species. Based on the expertise of preeminent fish ecologist Dr. Peter Moyle, the Accord required annual fall pulses to cue upstream migration of salmon (even though there were no salmon in the creek at that time), and a modest springtime pulse to support natural riparian processes, fish spawning, rearing, and outmigration.  Dry season baseflows were required to prevent streambed drying, and although wet season peak flows were not specified, winter high flows from tributaries and spill flows from Monticello Dam in wet years provide occasional scouring flows that clean and reshape stream habitat (Figure 1). The Accord did not require higher wet season baseflows relative to summer flows nor a spring recession that gradually transitioned from high spring flows to lower summer baseflows. Thus, the Accord flow regime implemented a Functional Flows approach “to a certain extent”, providing several functional flow components specified in CEFF.  

In our paper, we used a statistical approach that leverages long-term fish data to evaluate the response of native and non-native fish assemblages to each functional flow component. We found that as functional flow components shiftedtowards their predicted natural range, the likelihood of precipitous population declines (quasi-extinction risk, defined as a > 80% decrease over a ten-year period) decreased for the native fish assemblage.  However, functional flow components that shifted away from their estimated natural range increased quasi-extinction risk for native species (Figure 2). 

Essentially, the more the environmental flow regime approached a natural functional flow regime, the greater the likelihood that native fishes would succeed. In contrast, non-native species abundance decreased when flow components shifted toward predicted natural ranges and increased when components shifted away from their natural range. With respect to Accord flows, shifts in the dry season duration, fall pulse magnitude, wet season magnitude and timing, and spring recession timing towards more natural ranges contributed to native species recovery, while shifts in the spring flow and dry season baseflow magnitudes away from natural ranges may contribute to non-native species persistence. Although most functional flow components remained outside of their natural ranges following implementation of Accord flows, our results show that even moderate shifts toward a more natural flow regime benefited native fishes and generally suppress non-native fishes.

Overall, our study showed how implementing an environmental flow regime focused on seasonal, functional flow components derived from natural flows supports a robust, and still recovering, native fish assemblage that includes the return of Chinook salmon to Putah Creek.  Additional actions that increase functionality of seasonal water allocations seem likely to further support and enhance the native fish assemblage. 

So, “Is there empirical evidence that Functional Flows really support California native fish?” Our study results indicate “Yes”.  And, “Has a Functional Flows approach been implemented anywhere in the state?”, the answer is “to a certain extent”, in that the Putah Creek Accord required flows that provide several functional flow components. 

Given the success of the Putah Creek Accord and the scientific weight of evidence integrated into CEFF, we look forward to seeing the effects of further implementation of CEFF and a Functional Flows approach to impaired streams throughout California. 

Sarah Yarnell is a Senior Research Hydrologist at the Center for Watershed Sciences. Her research focuses on integrating the traditional fields of hydrology, ecology, and geomorphology in the river environment with application to sustainable water management. When she’s not in the office or teaching, you can find her outside studying and recreating in rivers and mountains near and far. Ethan Baruch is a Senior Environmental Scientist at the California Department of Fish and Wildlife, and former Postdoc at the Center for Watershed Sciences. Andrew L. Rypel is a Professor and the Peter B. Moyle and California Trout Chair of coldwater fish ecology at the University of California, Davis. He is a faculty member in the Department of Wildlife, Fish & Conservation Biology and past Director of the Center for Watershed Sciences. Robert A. Lusardi is an Assistant Professor in the Department of Wildlife, Fish, and Conservation Biology and Associate Director at the Center for Watershed Sciences.

Further Reading:

Baruch, EM, SM Yarnell, TE Grantham, JR Ayers, AL Rypel, and RA Lusardi. 2024. Mimicking functional elements of the natural flow regime promotes native fish recovery in a regulated river. Ecological Applications 34, no. 6 (2024): e3013.  https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/eap.3013

California Environmental Flows Framework (CEFF):  https://ceff.ucdavis.edu/

Grantham, T. et al. 2020. Functional Flows can improve environmental water management. https://californiawaterblog.com/2020/11/29/functional-flows-can-improve-environmental-water-management-in-california/

Jacinto, E. et al. 2023. Putah Creek’s rebirth: a model for other degraded streams? https://californiawaterblog.com/2023/07/08/putah-creeks-rebirth-a-model-for-reconciling-other-degraded-streams/

Obester, A. et al. 2020. Environmental Flows in California. https://californiawaterblog.com/2020/03/18/environmental-flows-in-california/

Parisek, C. et al. 2023. Schooling fish: behind the scenes of Putah Creek fish sampling. https://californiawaterblog.com/2023/11/19/schooling-fish-behind-the-scenes-of-putah-creek-fish-sampling/

Rabidoux, A. et al. 2022. The Putah Creek fish kill: learning from a local disaster. https://californiawaterblog.com/2022/04/24/the-putah-creek-fish-kill-learning-from-a-local-disaster/  

Rypel, A. 2022. Being patient and persistent with nature. https://californiawaterblog.com/2022/10/16/being-patient-and-persistent-with-nature/

Willmes, M. et al. 2020. New insights into Putah Creek salmon. https://californiawaterblog.com/2020/10/18/new-insights-into-putah-creek-salmon/  

Williams, J.G., Moyle, P.B., Webb, J.A. and Kondolf, G.M., 2019. Environmental flow assessment: methods and applications. John Wiley & Sons. https://www.wiley.com/en-us/Environmental+Flow+Assessment%3A+Methods+and+Applications-p-9781119217367

Yarnell, S. et al. 2018. Functional flows for developing ecological flow recommendatiions. https://californiawaterblog.com/2018/12/09/functional-flows-for-developing-ecological-flow-recommendations/

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