Managing salinity protects native fish species and beneficial uses of Delta water.

The Measure

Water management agencies comply with State Water Resources Control Board objectives for salinity in the Delta for D-1641 and US Fish and Wildlife's 2008 Biological Opinion for X2. 

  • Expectations

    Water management agencies comply with State Water Resources Control Board objectives for salinity in the Delta for D-1641 and X2.

  • Performance Metrics

    • Monthly electrical conductivity (EC), water temperature, and X2 in the Delta, evaluated annually.

Compliance Rate of Salinity Objectives in the Delta

Click on the points to view the compliance rates (percentage) and if the targets have been met for that station. This performance measure is evaluated annually. Some stations do not have data or did not have compliance values at that time and will appear as blank or at zero percent. (E.g. Station S-21, S-42)

Compliance reports of D-1641 salinity objectives can be viewed at the State Water Board's website. Suisun Marsh Salinity reports can be found on the California Natural Resource Agency data portal.

Salinity Compliance Stations

X2 in the Delta

The X2 targets for September and October come from the U.S. Fish and Wildlife Service (USFWS) Delta Smelt Biological Opinion Fall X2 Action to maintain the low salinity habitat at or less than 74 km in the fall following wet years and at or less than 81 km in the fall following above normal years.

X2 Locations
Transects along the axis of northern San Francisco Bay used to measure X2 in the UnTRIM Bay-Delta model

What is X2?

X2 is the distance from the Golden Gate to the point where daily average salinity is 2 percent at 1 meter off the bottom of the waterbed.

*2017 Relaxed Targets

From the USFWS memo: Relaxed targets of the 2008 BiOPs were approved by US Fish and Wildlife Service (USFWS). The Bureau of Reclamation operated its facilities to achieve an average X2 distance of no greater than 80 km instead of the 74 km for October 2017.

From the CDFW memo: California Department of Fish and Wildlife (CDFW) determined that the change to the implementation of the fall X2 Action in October 2017 was not consistent with the California Endangered Species Act. The Department of Water Resources committed to keep X2 at 79 km monthly average for October to significantly minimize any short-term habitat effects to delta smelt.

USFWS 2017 Memo  CDFW 2017 Memo 

Photo Source: Synthesis of studies in the fall low-salinity zone of the San Francisco estuary, September–December 2011: U.S. Geological Survey Scientific Investigations Report 2014–5041, 136 p.

Salinity in the Delta is an important water quality characteristic affecting municipal, industrial, agricultural, and fish and wildlife water uses. Changes in Delta salinity have far-reaching impacts, from affecting water supply for farmers in the Central Valley to the wildlife and ecosystems of the San Francisco Bay. When salinity exceeds compliance conditions or changes too rapidly, it can have negative impacts on many beneficial uses of water. This includes altered water taste and availability, crop damage and loss, and limitations for recharging groundwater.

The Delta is a transition zone between freshwater and saltwater, serving as an important habitat for hundreds of native terrestrial and aquatic organisms. The amount of freshwater flowing into and through the Delta drives seasonal and annual salinity levels.

Both natural and man-made actions affect salinity in the Delta and Suisun Marsh. Tidal forces, agricultural run-off, exports, and freshwater inflow from the Sacramento and San Joaquin Rivers jointly influence salinity levels of the Delta. The management of salinity is governed by the Department of Water Resources (DWR) and the U.S. Bureau of Reclamation (USBR) due to their roles in managing reservoirs in the watershed and water exports in the Delta. Since DWR and USBR are water right holders for the State Water Project and Central Valley Project, respectively, they must comply with State Water Board Decision 1641 requirements which help protect water quality in the Delta by setting flow and salinity objectives.

Each chapter of the Delta plan includes strategies to achieve the goals of the plan. These strategies are general guidance on achieving the objective laid out in the plan and in the Delta Reform Act of 2009. Associated with these strategies are recommendations. The recommendations describe more specific and implementable actions to support the achievement of Delta Plan strategies. Strategies and recommendations may also have associated performance measures. Delta Plan performance measures track progress in achieving desired outcomes for the Delta Plan. Below are the strategies and recommendations associated with this performance measure.

Delta Plan Strategy

  • Protect beneficial uses by managing salinity

Delta Plan Recommendations

  • Refer to Chapter 4's functional flows recommendations

Monthly electrical conductivity, water temperature, and X2 in the Delta and Suisun Marsh, evaluated annually. 


Average monthly electrical conductivity, water temperature, and X2 at SWRCB compliance points from 1995 to 2015.

  • Water management agencies meet SWRCB salinity objectives for ecosystem purposes at least 99 percent of the time at compliance points
  • Water management agencies meet all other SWRCB salinity objectives for urban and agricultural beneficial use at least 99 percent of the time at compliance points
  • Water management agencies maintain average X2 for September and October at or less than 74 km in the fall following wet years and at or less than 81 km in the fall following above normal years. The monthly average X2 must be maintained at or seaward of these values for each individual month and not averaged over the two-month period.
  1. Delta Compliance reports are available from DWR. These reports point out which stations went over the compliance value and for how long.
  2. The specific locations and objectives are located in SWRCB's Water Quality Control Plan Appendix K
  3. Salinity measurements can also be viewed at DWR's CDEC.
Data Sources
Delta Compliance
Water Quality Control Plan Appendix K
California Data Exchange Center

One important salinity standard is the “X2” factor and it is designed specifically to protect the aquatic life of the Delta estuary. X2 is a physical attribute of the estuary used as a habitat indicator for the location of the low salinity zone. X2 is the location in kilometers from the Golden Gate Bridge where water salinity is 2 ppt (parts per thousand) of isohaline salt.

The low salinity zone is where freshwater transitions into brackish water. Historically, the low salinity zone was associated with high primary productivity, zooplankton population, and abundance of native species. U.S. Fish and Wildlife Service designated an additional X2 standard for fall months to provide suitable habitat for organisms using this low salinity region, and is implementing an adaptive management plan for this action to study the relationship between fall X2 and the endangered Delta smelt.

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