By Lindsay Murdoch, Sarah Yarnell, and Jay Lund

California’s local communities and native ecosystems alike have adapted to cycles of flood, drought, and a healthy portion of everything in between. Our river management, on the other hand, has fallen out of natural balance and tends to oscillate between insufficient minimum flows and emergency flood responses, missing much of what our rivers need most: the variability in between. 

Variability should not be approached as a crisis to manage at the extremes, but rather as a rhythm to embrace. The functional flows approach helps us quantify environmental water needs in a way that embraces these natural rhythms, which express themselves as seasonal patterns that support a wide range of physical and biotic functions from year to year (Fig. 1, Yarnell et al., 2015). Restoring these functional flows is a way to preserve more natural patterns, while leaving some remaining water available each year for other beneficial uses, such as agricultural and urban water supplies. 

 

The natural flow regime (e.g., Fig. 2) encompasses the entire spectrum of flows in an unregulated, unimpaired river, ranging from the most extreme wet years to the deepest droughts (and everything in between).  The natural flow regime has some common seasonal patterns, while year-to-year differences are stark (interannual variability). The heterogeneity of natural systems includes both the river’s seasonal and interannual variability—acting in tandem to support complex ecological dynamics. Both types of variability should be considered in any environmental flow prescription; no single annual flow pattern will provide the full range of flows that are necessary to support native ecosystems.

This regime-based thinking is extendable to functional flow operation of rivers (Murdoch 2024).  A functional flow “regime” is the spectrum of flows that support the historical range of flow-driven functions while preserving their natural return intervals. The regime serves as an implementation tool that encompasses natural seasonality and interannual variability, preserving the spectrum of flow-driven functions. Just as understanding functional flows shifted our framework away from establishing one minimal flow level and toward establishing a dynamic set of flows over the course of a year, understanding functional flow regimes shifts us away from rigid annual flow and toward interannual variation in functional flow requirements.  Restoring such functional flow regimes might be our best chance at restoring a river’s ability to regulate itself.  

Developing a functional flow regime in four easy steps

Step 1. Quantify functional flow metrics

A thorough analysis of the natural flow regime is an ideal starting place for understanding how our rivers historically functioned. Unfortunately, we are most often interested in developing functional flow regimes for impaired systems with significant flow deviations from natural conditions. In the absence of natural flow data, the best alternative is to use estimates of unimpaired flows. Various methods exist for estimating daily unimpaired flow (outside the scope of this blog), but it is crucial to understand the limitations of the chosen estimation method. 

Functional flow metrics for California rivers can be readily computed from historical daily unimpaired flow using the Functional Flows Calculator (publicly available via https://flowcalculator.codefornature.org/). The calculator outputs a set of descriptive metrics, including magnitude, timing, duration, and rate of change, that describe the functional flow components for each year of unimpaired flow data. (We love data, especially when there’s a lot of it.) 

Step 2. Adjust metrics for non-flow limitations, as needed

Here, we pause to consider whether the natural range of each metric, observed across years, is desirable in the existing riverscape. We might flag (naturally) high magnitude metrics that could threaten public safety and property. It would be a shame to intentionally flood Sacramento, the historical floodplain where I live. We might also flag pulse flow timings that could endanger fragile redds, now located well downstream of their historical spawning grounds. Other considerations include altered channel morphology (e.g., floodplain connectivity), sediment availability, riparian composition, and restoration efforts, as well as chemical and temperature conditions, reservoir operation limitations, and changes in the timing of water availability due to climate change. 

This step can be overwhelming and should be approached iteratively, in keeping with iterative learning and adaptive management. 

Step 3. Relate metrics to water year percentiles

Managing flows by a few water year types is soooo 20^th century. We can, and must, do better. Water year percentiles—calculated by ranking annual unimpaired flows by volume—convey a more complete picture of interannual variability. By this metric, the driest years have low water year percentile values (e.g., 0.05 and 0.23), while the wettest years have high percentile values (e.g., 0.85 and 0.99). Water year percentiles are particularly desirable because they preserve the interannual frequencies of these years. Annual flow volumes tend to follow skewed distributions, with fewer extreme high flows that have a wide range (consider commonly used 100- and 1000-year return intervals used by the Army Corps to characterize infrequent high flow events). 

When flows vary, metrics vary. Some metrics (particularly magnitude metrics) have very strong correlations with water year percentile. By relating metrics to corresponding water year percentiles, we can develop relationships to represent how these metrics vary in wetter and drier years. This paints a nuanced picture of how seasonal patterns change from year to year. 

These relationships are flexible and can be structured around constraints identified in Step 2—though managing tradeoffs resulting from deviations from the natural flow requires creativity and thoughtfulness. 

Step 4. Assemble Functional Flow Hydrographs 

Given any water year percentile, we can use relationships developed in Step 3 to assess year-appropriate functional flow metric values. Those metric values are then used to formulate unique hydrographs that reasonably represent the functional signature of that water year percentile. 

A more complete functional flow regime emerges by assembling these hydrographs across water year percentiles—visualized as a spectrum of hydrographs that represent natural seasonality and interannual variability in identified functional flows (Fig. 6). 

Applications of Functional Flow Regimes

Functional flow regimes built on water year percentiles have a broad potential for planning, policy, and improving our scientific understanding of rivers. Already, these ideas are gaining traction and being applied in multiple research efforts:

1. Simulating real-time flow operation using existing forecasts to determine environmental water scheduling under uncertainty (Yarnell et al. 2024; Murdoch 2024, Murdoch et al. 2025). 
2. Crafting water allocation policies that link allocations with year-to-year environmental needs to match patterns of interannual variability (Murdoch 2025).
3. Assessing alteration in patterns of interannual variability (Murdoch 2024). 
4. Bridging single-species and holistic flow management approaches, by coupling objective-oriented flow features to protect winter- and spring-run salmon with functional flow patterns (NOAA Fisheries).
5. Simulating how system-wide functional flow operations impact water supply and deliveries under different possible climate futures and water system configurations (COEQWAL; Yi et al., upcoming)
6. Simulating environmental needs and assessing alteration under different management planning scenarios (DWR).  

The California Environmental Flows Framework’s Technical Report provides a roadmap for producing implementation-ready functional flows. The long-term success of these efforts relies on the involvement of diverse stakeholders and participation from local communities. There are many tradeoffs and compromises to consider. Regime-focused environmental flows are a promising approach and framework for adaptively considering and managing these complex puzzles, thereby improving the operation of river and stream flows for native ecosystems. 

About the Authors

Lindsay Murdoch is a PhD Candidate in Water Resources Engineering and a student researcher at the Center for Watershed Sciences. Her work is application-focused and integrates modeling, planning, and decision support for environmental flow management. Before UC Davis, Lindsay worked as a coordinator for a peer-to-peer learning network for watershed restoration practitioners in the arid West. 

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.

Jay Lund is an Emeritus Distinguished Professor of Civil and Environmental Engineering and Vice Director of the Center for Watershed Sciences at the University of California – Davis.

Further Reading

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

Murdoch, LE. (2024). “Adaptively Operating a Fixed-percent Environmental Flow Budget with a Functional Flows Approach”. MS thesis. University of California, Davis.

Murdoch, LE, SM Yarnell, F Bellido-Leiva, C Carpenter,L Calvo, & J Lund. (2025). “Functional Flows Adaptive Implementation Method (FFAIM): Insights from Three Lower San Joaquin River Tributary Case Studies”. Technical Report for California State Water Resources Control Board. Available by request: lemurdoch@ucdavis.edu.

Yarnell SM, Petts GE, Schmidt JC, Whipple AA, Beller EE, Dahm CN, Goodwin P, Viers JH. (2015). Functional Flows in Modified Riverscapes: Hydrographs, Habitats and Opportunities. Bioscience 65:963-972. https://doi.org/10.1093/biosci/biv102

Yarnell SM, Stein ED, Webb JA, Grantham T, Lusardi RA, Zimmerman J, Peek RA, Lane BA, Howard J, 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. https://doi.org/10.1002/rra.3575

Related California WaterBlog links 

Yarnell, S., A Obester, T Grantham, E Stein, B Lane, R Lusardi, J Zimmerman, J Howard, S Sandoval-Solis, R Henery, and E Bray (2018). Functional Flows for Developing Ecological Flow Recommendations. California WaterBlog.

Grantham, T., J Howard, B Lane, R Lusardi, S Sandoval-Solis, E Stein, S Yarnell, and J Zimmerman. (2020). Functional Flows Can Improve Environmental Water Management in California. California WaterBlog. 

Yarnell, S., D Rivera Salazar, C Boettiger, and J Lund. (2024). A Functional Flows approach for Environmental Flows in Chile. California WaterBlog.

Stanford B., J Zimmerman, K Taniguchi-Quan, T Grantham, S Yarnell, A Obester, E Stein, J Ayers, A Milward. (2024). A Functional Flows approach to implementing Flood-MAR. California WaterBlog.

Yarnell, S., E Baruch, AL Rypel, and R Lusardi. (2025). Functional Flows are Good for California’s Native Fishes.California WaterBlog.