Background
The approximately 59,000-acre Yolo Bypass and approximately 18,000-acre Sutter Bypass are essentially leveed floodplains designed to convey floodwaters from the Sacramento River, Feather River, west-side streams and agricultural drains around the populated regions of the central Sacramento Valley. During high outflow events, rising floodwaters overtop the weirs and inundate the floodplains, which are bounded by a system of levees that serve to contain the floodwaters. The Yolo Bypass conveys up to 80 percent of the system’s floodwaters for a distance of approximately 31 miles before discharging back into the Sacramento River upstream of Rio Vista. Yolo Bypass flows attract upstream migrating fish from the Sacramento River into the Yolo Bypass at the mouth of the Cache Slough complex near Liberty Island. Among these are several federal and state listed species including Sacramento River winter-run Chinook salmon (federal and state endangered), Central Valley spring-run Chinook salmon (federal and state threatened) Central Valley steelhead; (federal threatened), and the southern distinct population segment (DPS) green sturgeon (federal threatened). These fish continue migrating up the Yolo Bypass and may be able to return to the Sacramento River via the Fremont Weir or Tisdale Weir in the Sutter Bypass as long as a sufficient volume of water flows over the weirs. However, when floodwaters recede, numerous fish can be trapped in the weir splash basins and downstream scour ponds, pools, and swales. Even when floodwaters do not overtop flood control weirs, flows entering the Cache Slough complex from the west side streams and agricultural drains are oftentimes sufficient to attract fish into the Yolo Bypass. These fish continue their upstream migration through a series of perennial flowing agricultural drains such as Toe Drain, Tule Canal and Knights Landing Ridge Cut. A newly installed fish collection facility at
Wallace Weir will facilitate the efficient and timely return of fish migrating up the Tule Canal (see Wallace Weir tab). However; greater numbers of adult fish, particularly listed species such as winter-run and spring-run Chinook salmon, southern DPS green sturgeon, and Central Valley steelhead enter the Yolo Bypass during overtopping of the Fremont and Tisdale Weirs and subsequent inundation of the Yolo Bypass.
CDFW staff has conducted numerous fish rescues at the Fremont and Tisdale weirs for over 60 years, saving tens of thousands fish including numerous individuals of the aforementioned listed species. The recently issued Draft Environmental Report for the Yolo Bypass Salmonid Habitat Restoration and Fish Passage Project addresses alternatives to modify the Fremont Weir to increase volitional fish passage back to the Sacramento River. However, the project may not be completed until 2021. Although project completion is expected to enhance fish passage and reduce stranding in the Yolo Bypass, CDFW will continue to monitor the Yolo Bypass for fish stranding and conduct rescues as necessary. As there are no immediate plans to enhance fish passage at Tisdale Weir, CDFW staff will continue rescue operations at Tisdale Weir.
Description
View of Fremont Weir Denil fish ladder during overtopping event in January 2006.
Flash boards are left in place until overtopping of the weir ceases.
When Sacramento River flows overtop the Fremont and Tisdale weirs, CDFW staff begin to plan for upcoming fish rescue operations. Once overtopping of the weirs ends, CDFW staff conduct reconnaissance surveys to determine the number and species composition of stranded fish. Timing of fish rescue operations is dependent on factors such as water depth temperature, inundation area, species composition, and potential safety issues. CDFW staff then develop a work plan including staffing and equipment needs. Efficient fish rescue operations are typically conducted by a well-trained staff of eight to 12 or more personnel and utilize various fisheries equipment includes specially designed crowder racks for adult fish entrained in the weir aprons, beach seines, dip nets, sturgeon hoop nets, backpack electro-shockers, aerators, ice chests, and 150-gallon tubs. All fish captured are identified to species, enumerated, assessed for condition, and measured. If large numbers of fish are encountered, a subset of 20 length measurements for each species is recorded. Adult green sturgeon and adult white sturgeon are surgically implanted with VEMCO® V16 acoustic transmitters, steelhead are surgically implanted with passive integrated transistor (PIT) tags, and adult salmonids are tagged with two external anchor tags prior to release back into the Sacramento River. Tagging and subsequent monitoring of rescued fish provides data used to determine post-entrainment survival and behavioral characteristics of adult sturgeon and salmonids. Tagged adult sturgeon are tracked via an array of acoustic receivers deployed throughout the Sacramento River, Sacramento-San Joaquin Delta, and San Francisco Bay. Tagged adult and juvenile steelhead are monitored by PIT tag antenna arrays deployed in known spawning streams and at fish hatcheries. Recovery of anchor tags by fish hatchery personnel, anglers, and escapement (carcass) surveys provide data regarding Chinook salmon post-rescue survival and spawning success.