Estimated Inundation Periods in the Yolo Bypass, California, 1998 – 2024

Largely supported by the Interagency Ecological Program (IEP), California Department of Water Resources (DWR) has operated a fish monitoring program in the Yolo Bypass, a seasonal floodplain and tidal slough, since 1998.
The objectives of the Yolo Bypass Fish Monitoring Program (YBFMP) are to:

1. Collect baseline data on water quality, chlorophyll, lower trophic level biota, and fish in the Yolo Bypass to monitor spatial and temporal changes in trends and abundance.
2. Analyze and communicate Yolo Bypass data with interested parties and the scientific and management communities to address pertinent management-related questions.
3. Provide technical expertise on Yolo Bypass aquatic ecology and monitoring and sampling methods.
The YBFMP operates a rotary screw trap and fyke trap and conducts biweekly beach seine and lower trophic surveys in addition to maintaining water quality instrumentation in the bypass.
The YBFMP serves to fill information gaps regarding environmental conditions in the bypass that trigger migrations and enhanced survival and growth of native fishes, as well as provide data for IEP synthesis efforts. YBFMP staff also conduct analyses of YBFMP monitoring data to address pertinent management related questions as identified by IEP. The Yolo Bypass has been identified as a high restoration priority by the National Marine Fisheries Service and US Fish and Wildlife Service Biological Opinions for Delta Smelt, Winter and Spring-run Chinook salmon and by California EcoRestore. The YBFMP informs the restoration actions that are mandated or recommended in these plans and provides critical baseline data on the ecology of the bypass and how it interacts with the broader San Francisco Estuary.
YBFMP’s data is often accompanied by information on whether the Yolo Bypass is inundated, as water quality, and species composition and abundance can be greatly altered during inundation. This dataset was created to consistently estimate inundation over time. Estimating inundation in the Yolo Bypass is usually done by referencing stage height in the Sacramento River at Fremont Weir. Stage height is the water level of the river. Fremont Weir is upstream of the Yolo Bypass and when Fremont Weir overtops, or reaches its monitoring stage, the Yolo Bypass is considered inundated.