Pick Our Brain - September 2011

What is “X2” and why is it important?

Like all estuaries, the Bay-Delta is a place where freshwater mixes with saltwater. Where this transition from freshwater to saltwater takes place and how it changes through time is hypothesized as an important factor in aquatic ecosystem health. One measure of this freshwater-saltwater transition is a location in the estuary called X2. (“X” refers to distance. The number “2” refers to the amount of dissolved salts in the water in parts per thousand.) This is the location in the Bay-Delta where the tidally averaged bottom salinity is 2 parts per thousand. It is expressed as the distance in kilometers (km) from the Golden Gate Bridge.

The location of the freshwater-saltwater interface (X2) along the upstream-downstream axis of the estuary shifts with the seasons and from year to year depending on the amount of precipitation and Delta outflow (the amount of freshwater flowing from the Delta into Suisun Bay and then San Francisco Bay). Higher Delta outflow pushes saltwater further towards the Bay, lower Delta outflow allows the freshwater-saltwater interface to move upstream into the Delta.

The following map shows where various values of X2 are located relative to Bay-Delta geographic features.

Final Environmental Impact Report for the Implementation of the 1995 Bay-Delta Water Quality Control Plan
From the State Water Resources Control Board
Final Environmental Impact Report for Implementation of the 1995
Bay-Delta Water Quality Control Plan
Four month chart of X2 (freshwater-saltwater transition location) in 2011
Four Month Chart of X2 in 2011, from DWR California Data Exchange Center (CDEC)

The chart shows the location of X2 for the four months ending September 7, 2011.

Note that X2 is low (meaning the saltwater-freshwater interface is closer to the Golden Gate Bridge) in the spring, and high in the summer (farther from the Golden Gate Bridge).

X2 values vary with the seasons, but in general, lower X2 values (i.e., higher Delta outflows) are correlated with higher abundances of delta smelt, longfin smelt, Bay shrimp and other estuarine species. The most recent Endangered Species Act regulations require certain values of X2 in the fall after above normal and wet years.