by Francheska Torres, Miranda Tilcock, Alexandra Chu, and Sarah Yarnell
The Yolo Bypass is one of two large flood bypasses in California’s Central Valley that are examples of multi-benefit floodplain projects (Figure 1; Serra-Llobet et al., 2022). Originally constructed in the early 20th century for flood control, up to 75% of the Sacramento River’s flood flow can be diverted through a system of weirs into the Yolo Bypass and away from nearby communities (Figure 2; Salcido, 2012; Sommer et al., 2001). During the dry season, floodplain soils in the bypass support farming of seasonal crops (mostly rice). Today, the bypass is also widely recognized for its ecological benefits. In 1994, much of the bypass was designated as a Wildlife Area by the Fish and Game Commission, with the goal of reestablishing wetland habitat for waterbirds along with other wildlife (Yolo Bypass Wildlife Land Management Plan, 2008). The Yolo Bypass is one potential representation of harmony that can be achieved between floods, farming, fish, and feathers (Salcido, 2012).
The Yolo Bypass supports a range of broad multi-benefits to ecosystems and society:
During winter when high runoff fills rivers, the bypass provides space for floodwaters to spread and travel downstream to the Delta without damaging homes or communities. During high precipitation events, excess water enters the Yolo Bypass at Fremont Weir when Sacramento River flows exceed ~55,000 cubic feet per second (Sommer et al., 2001). When the Sacramento River reaches 27.5 feet at Sacramento’s I Street Bridge, the Sacramento Weir can be opened manually for additional water to drain into the Yolo Bypass, and eventually into the bypass’ Toe Drain near Rio Vista. During storms, you can check if flows over-top the Fremont Weir, using the California Data Exchange Center (CDEC) at https://cdec.water.ca.gov/guidance_plots/FRE_gp.html
In dry months (late spring and summer), the Yolo Bypass supports farming of seasonal crops including rice, safflower, processing tomatoes, corn, and sunflower. Wild rice has a tolerance to colder weather and is one of the types of rice grown in the bypass (Sommer et al., 2011). In winter (Dec-Mar), the existing field infrastructure can extend the duration of water inundation to facilitate development of invertebrate biomass (Corline et al., 2017), growing food to help struggling fish populations. Leftover crop residue from harvested lands supports fish and wildlife as a foraging area.
During winter when flooding occurs, the Yolo Bypass becomes a food-rich habitat for many fish species, including Chinook Salmon. Floodplains are important nursery habitats for juvenile salmon in California, providing growth opportunities from a highly productive food web (Jeffres et al., 2020). Juvenile salmon rearing on the floodplain can grow up to 1mm a day (Corline et al., 2017; Katz et al., 2017; Rypel et al., 2022)! For small salmon, growth rates provide huge benefits for their journey to the ocean and later survival.
The Yolo Bypass is part of the Pacific Flyway, one of four primary migration routes through North America for birds, particularly waterfowl (Bird et al., 2000; Eadie et al., 2008; Sommer et al., 2011). Each year, many bird species migrate through the Yolo Bypass or use this area for nesting. The Yolo Bypass Wildlife Area (YBWA), in particular, is a success story for shorebird habitat and productive waterfowl (Salcido, 2012). The Swainson’s hawk, a threatened species, frequents YBWA, with up to 70 individuals observed foraging on the floodplain at once (Sommer et al., 2011). YBWA also provides recreational (bird watching and hunting) and educational opportunities.
Floodwaters spread across the bypass, seep into the subsurface recharging groundwater, and fill local shallow aquifers. Nitrogen and phosphorus are also delivered and infiltrate floodplain soils subsequently assisting plant growth. Groundwater provides a key water source during dry summers and times of drought, with roughly 85% of Californians relying on groundwater as a source of drinking water (Harter, 2008). In California approximately 40% of water demand is met by groundwater (Figure 3).
Management goals for the Yolo Bypass have expanded from flood management and agriculture to include habitat management and restoration for birds and fishes (Serra-Llobet et al., 2022). This current, multipurpose, version of the Yolo Bypass is a model of an increasingly well-managed multi-benefit social-ecological system with public-private partnerships that allows wildlife, flood risk reduction, and agriculture to co-exist adjacent to a major urban region. Its potential to provide greater inundated floodplain habitat with more natural patterns of inundation is widely recognized, with expanding benefits for nature and humans. Documentation of the remarkable ecological value of the inundated bypass has helped to shepherd a new emphasis on floodplain restoration throughout the Sacramento-San Joaquin Valley (Johnson, 2017).
Franceska Torres is a Junior Specialist at the Center for Watershed Sciences studying otoliths and what they can tell us about salmon migration, their age and their growth. She got her bachelor’s degree in Marine and Coastal Science with an emphasis in Marine Ecology and Organismal Biology from the University of California, Davis. Miranda Bell Tilcock is an Assistant Specialist at the Center for Watershed Sciences. Alexandra Chu is a Junior Specialist at the Center for Watershed Science. She works on the Eyes and Ears Project, peeling eye lenses from Chinook Salmon for stable isotope analysis to reconstruct their life history and identify critical rearing habitats. Sarah Yarnell is an Associate Professional Researcher at the Center for Watershed Sciences. Her studies focus on integrating the traditional fields of hydrology, ecology and geomorphology in the river environment.
Bird, J. A., Pettygrove, G. S., & Eadie, J. M. (2000). The impact of waterfowl foraging on the decomposition of rice straw: mutual benefits for rice growers and waterfowl. Journal of Applied Ecology, 37(5), 728–741. https://doi.org/https://doi.org/10.1046/j.1365-2664.2000.00539.x