The Department of Water Resources has released a report summarizing the construction, monitoring, effectiveness, and impacts of the 2021–2022 West False River Emergency Drought Salinity Barrier. This report was prepared to fulfill the reporting requirement by the State Water Board’s May 2021 Water Quality Certification and the commitment detailed in the DWR’s Monitoring Plan.
In spring 2021, DWR was authorized to install an emergency drought barrier to manage critically low water supplies due to forecasted drought conditions in multiple consecutive years, low levels of reservoir water storage, the high risk of exceeding water quality objectives, and the results of drought modeling and monitoring.
The barrier was installed on the West False River in the Delta, approximately 5 miles south of Rio Vista near Franks Tract. This location was selected based on hydrodynamic modeling of salinity patterns in the Delta, which determined that the location was optimal for placing a single barrier. It was also the location for the 2015 drought salinity barrier. The installation was completed in June 2021.
The emergency drought barrier was a temporary physical rock-fill barrier that reduced the intrusion of high-salinity water into the Central Delta, similar to the drought salinity barrier installed in 2015. However, the emergency drought barrier was kept in place through November 2022 instead of being removed in the fall of 2021; the 2015 barrier was removed in the fall of that year.
Goals and objectives of the barrier
The Delta is connected to San Francisco Bay and is influenced by tides twice daily pushing in and out from the Bay; outflow from rivers and/or reservoir releases push back the tides to keep saltwater from intruding into the Delta. If saltwater intrudes into the Central Delta, the water can become unusable for Delta agriculture, local municipal supplies, and the environment, as well as the water supplies for export.
Generally, the State Water Project and Central Valley Project manage Delta outflow to keep the saltwater from intruding into the Delta. However, during severe droughts, storage in upstream reservoirs and natural inflows to the Delta may be insufficient to push back on the tides. This presents a difficult water management challenge, requiring measures such as the installation of the emergency drought barrier at West False River to minimize saltwater intrusion.
The emergency drought barrier was intended to help prevent saltwater intrusion, protect water quality, and reduce the amount of water released from reservoirs upstream to keep saltwater out of the Central Delta.
Monitoring was conducted to assess the effectiveness of the barrier in preserving water quality and rerouting salty water away from the South Delta and to assess the impact on ecosystem factors, including bathymetry and channel bed elevation, salinity, nutrients, turbidity, phytoplankton, zooplankton, harmful algal blooms, aquatic weeds, and fish predation. The methods, approaches, analysis, and results are discussed in extensive detail in Chapter 2 of the report.
Findings
Overall, the 2021–2022 EDB achieved its objective of reducing salinity in the Central Delta while preserving upstream storage, but at best, it was only a modest influence at the State Water Project and Central Valley Project pumps. The barrier altered tidal dispersion patterns, so less saline water was pumped into the South Delta through Franks Tract. However, this caused higher tidal energy on the Sacramento and San Joaquin rivers, which caused salinity to intrude near San Andreas Landing, a D-1641 compliance point on the San Joaquin River.
Models suggest there are advantages to using flow management rather than the barrier when water supply allows; the report suggests the benefits of the barrier in medium-flow conditions should be evaluated, as the barrier is a significant engineering project requiring a lengthy planning and implementation process.
The most substantial impact of the emergency drought barrier on flow occurred within Franks Tract itself. Water age increased significantly on the western side and decreased slightly on the eastern side; velocity increased significantly through Fisherman’s Cut and Old River at Franks Tract while decreasing at Holland Cut and Quimby Island. These changes to the flow might have contributed to a large harmful algal bloom in Franks Tract during July and August of 2021.
With the barrier left in place over the winter season, DWR constructed a temporary notch in the center of the barrier from January 2022 to March 2022 to facilitate fish and boat passage. The notch did allow for fish and boat passage, but it also resulted in a large scour hole at the bottom of the channel near the barrier’s location. DWR plans to evaluate the information available at the time to determine whether a notch should be placed in the future.
The 2021 installation represented the second time in seven years that drought conditions required an emergency barrier at the West False River location. Therefore, planning for the installation of a drought barrier on a non-emergency basis would be very beneficial to regional water management. These plans are already underway.
Recommendations and lessons learned
Overall, the emergency drought barrier achieved its objective of reducing salinity in the Central Delta while preserving upstream storage. Tracking fish passage near the barrier was challenged by a lack of receivers for tracking tagged fish and a limited number of tagged fish in the area despite thousands being released in the upper watershed. Deploying receivers near the barrier and using detections of fish tagged from other studies to the extent possible would increase sample sizes and help to better understand fish passage in the region.
Lessons learned include enhancing coordination before the decision to install the drought barrier, identifying and purchasing advance mitigation for future installations, and providing ample time to complete project closeout activities.
