by Sarah Yarnell, Alyssa Obester, Ted Grantham, Eric Stein, Belize Lane, Rob Lusardi, Julie Zimmerman, Jeanette Howard, Sam Sandoval-Solis, Rene Henery, and Erin Bray
To protect California’s native aquatic species, stream flows need to be managed to support important ecological processes and habitat needs.
In practice, such flows are difficult and controversial to define and implement. Water diversions, dams and other water infrastructure, land drainage, and changing climate conditions have altered the timing and availability of water, creating demands that impair restoration of the full natural flow regime. While restoring full natural flow regimes in California rivers may not be possible, preserving key aspects of the flow regime, or functional flow components, may provide a means to conserve the state’s freshwater ecosystems.
What are functional flows?
The functional flows approach provides a basis for estimating how much water is needed for the environment, where key components of the natural flow regime are targeted rather than the full natural flow regime. Desirable functional flow components have a disproportionately important role in supporting the physical and ecological processes that create and maintain habitat and trigger native species to reproduce, thrive, and migrate.
This process-based approach preserves the most important aspects of the variability of a natural flow regime to which native species have adapted. It differs from environmental flow methods that focus on single species and their habitat requirements by instead capturing the needs of biological communities.
Most rivers in California have four functional flow components of the natural flow regime represent significant drivers of geomorphic and ecological processes:
- Wet-season initiation flows, or the first major storm event following the dry season. These flows represent the transition from dry to wet season, and serve important functions such as moving nutrients downstream and signaling species to migrate or spawn.
- Peak magnitude flows, which transport a significant portion of sediment load and maintain and restructure river corridors.
- Spring recession flows, which signify the transition from high to low flows and provide reproductive and migratory cues and redistribute sediment.
- Dry-season low flows, which favor native species adapted to withstand stressful periods.
Each of these four components are quantified by flow characteristics, including magnitude, timing, duration, frequency, and rate of change. Together, these aspects of flow comprise a functional flow regime; no single component or characteristic alone constitutes a functional flow regime. The relative importance of functional flow components may vary locally, while flow characteristics will likely vary based on water year type (dry, moderate, wet conditions). Year-to-year hydrologic variability provides periodic disturbances needed to support diverse aquatic and riparian communities. Additionally, characteristics should remain similar to natural values observed historically in order to protect native species and ecological processes.
While the functional flows approach can help estimate environmental water needs, it may not by itself provide a solution to ecosystem recovery. Additional physical habitat restoration may be required to realize the full benefits of a functional flow regime. For example, channel floodplain connections may be needed to support benefits of high flows that provide habitat diversity. Additional management measures may be needed to address a spectrum of water quality concerns in a watershed, including stream temperature, nutrients, sediment, and dissolved oxygen. These factors should be considered and explored further when utilizing a functional flows approach. Chapman et al. (2018) illustrate the success of this type of approach for Putah Creek, California.
How could this approach be applied in a management context?
A recent PPIC report recommended establishing an ecosystem water budget, which would allocate a block of water to the environment. Such budgets would bring the environment to the table as a partner in water management and allow for the trustee responsible for managing that water budget to participate in buying and selling water.
The functional flows approach provides a strategy for defining and allocating ecosystem water budgets. For example, by quantifying the functional flow components for a particular system, they can be aggregated to estimate an annual ecosystem water budget. Once budgets are established, they can also be used to guide when environmental water allocations are necessary to achieve targeted flow functions, for example by releasing water from dams at critical times to increase peak winter flows, or by curtailing water diversions to ensure that summer baseflows remain within the targeted range required for ecosystem health. An ecosystem water budget informed by a functional flows approach also offers flexibility during changing conditions (wet and dry years) by providing context to consider seasonal or interannual differences in allocations that can balance ecological and human use.
The functional flows approach offers a flexible means of informing flow recommendations that capture significant processes upon which native species are hypothesized to depend. This approach could be an effective way to manage an ecosystem water budget for California that preserves important components of the natural flow regime.
Sarah Yarnell and Alyssa Obester are affiliates of the UC Davis Center for Watershed Sciences. Ted Grantham is faculty at the University of California, Berkeley, and an affiliate of the Center for Watershed Sciences. Eric Stein is a Principal Scientist at Southern California Coastal Water Research Project. Belize Lane is faculty at Utah State University. Rob Lusardi is also a researcher at the Center for Watershed Sciences and is the UC Davis-California Trout Coldwater Fish Scientist. Julie Zimmerman and Jeanette Howard are affiliated with the Nature Conservancy. Samuel Sandoval is an Associate Professor in the Dept. of Land, Air and Water Resources at UC Davis and an UC Agricultural and Natural Resources Cooperative Extension Specialist. Rene Henery is with Trout Unlimited. Erin Bray is faculty at California State University, Northridge. These individuals are continuing to work on implementing a functional flows approach across California via the Environmental Flows Workgroup, a sub-group of the California Water Quality Monitoring Council. Stay tuned for upcoming related blogs.
Chapman, E., E. Jacinto, and P. Moyle (2018). Habitat Restoration for Chinook Salmon in Putah Creek: A Success Story. California Water Blog.
Jeffres, C. (2011). Frolicking fat floodplain fish feeding furiously. California Water Blog.
Mount, J., Gray, B., Chappelle, C., Gartrell, G., Grantham, T., Moyle, P., Seavy, N., Szeptycki, L., and Thompson, B. (2017). Managing California’s Freshwater Ecosystems: Lessons from the 2012–16 Drought. Public Policy Institute of California.
Poff N.L., Allan J.D., Bain M.B., Karr J.R., Prestegaard K.L., Richter B.D., Sparks R.E., Stromberg J.C. (1997). The natural flow regime. BioScience 47: 769–784.
Yarnell, S.M., Stein, E.D., Lusardi, R.A., Zimmerman, J., Peek, R.A., Grantham, T., Lane, B.A., Howard, J., and Sandoval-Solis, S. (2018). An ecologically based approach for selecting flow metrics for environmental flow applications. In Review. Journal of Ecohydraulics.
Yarnell, S. M., Petts, G. E., Schmidt, J. C., Whipple, A. A., Beller, E. E., Dahm, C. N., … Viers, J. H. (2015). Functional Flows in Modified Riverscapes: Hydrographs, Habitats and Opportunities. BioScience, biv102. https://doi.org/10.1093/biosci/biv102
Willis, A. (2018). The folly of unimpaired flows for water quality management. California Water Blog.