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Report · August 2020

Making the Most of Water for the Environment: A Functional Flows Approach for California’s Rivers

Ted Grantham, Jeffrey Mount, Sarah Yarnell, and Eric D. Stein

Supported with funding from the S. D. Bechtel, Jr. Foundation and the funders of the PPIC CalTrout Ecosystem Fellowship


In California, water and land management activities have substantially altered river flows and degraded river channels and their floodplains. The result has been a precipitous decline in native fish populations, including the collapse of valued salmon fisheries, and widespread imperilment of freshwater biodiversity. Environmental flows-flows in rivers and streams necessary to sustain ecosystem health-are essential to reversing these trends. Yet many rivers in California lack environmental flows. And for those rivers with such protections, environmental flows are typically established at static minimum levels, which fail to preserve the natural seasonal and interannual variability of flow that sustains healthy ecosystems. A new approach to managing environmental water is needed.

Building on previous PPIC work in this area, we recommend a “functional flows” approach for managing water for the environment. Functional flows refer to components of a river’s flow that sustain the biological, chemical, and physical processes upon which native freshwater species depend. A functional flows approach does not mandate the restoration of natural flows or the maintenance of historical ecosystem conditions, but rather focuses on preserving key functions’such as sediment movement, water quality maintenance, and environmental cues for species migration and reproduction-that maintain ecosystem health. This approach also recognizes that suitable physical habitat is necessary to support the functions of flowing water.

By coupling physical habitat improvements with key aspects of flow variability, functional flows offer a more effective means of improving ecosystem health than conventional approaches. Managing environmental water as functional flows can also benefit people. A consistent, transparent, and science-based approach gives greater assurances to the public that investments in environmental water are justified. Resulting improvements in ecosystem health would also enhance fishing and recreational opportunities, as well as limit the risk of new Endangered Species Act listings and further regulatory restrictions on water users.

Elements of the functional flows approach have been put into practice in a few places, but implementation is in a nascent stage. Pilot studies and experimentation are needed to evaluate effectiveness and refine the approach. This means that robust, well-funded science and monitoring programs are essential to its success. Looking ahead, we suggest the functional flows approach can serve as a valuable tool for improving the health of freshwater ecosystems and building the long-term resilience of California’s water management system.


Drought Freshwater Ecosystems Water, Land & Air