X2 is the location in the Bay-Delta where the tidally averaged salinity is 2 parts per thousand.
In an estuary, the area where fresh water meets and mixes with salt water comprises the estuarine habitat that supports multiple life stages for a diversity of fishes. This transition from salt to fresh water and how it changes through time is thought to be an important factor in estuarine health. One measure of this transition used in the Delta is “X2”, which 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 X2 shifts depending on precipitation and the amount of Delta outflow. Higher Delta outflow will push the saltwater further towards the Bay, while a lower amount of Delta outflow allows the freshwater-saltwater interface to move upstream into the Delta. The location of X2 also shifts seasonally, generally being closer to the Golden Gate Bridge in the spring and farther from the Golden Gate Bridge in the fall. The location of X2 also shifts with the daily tides, so the position of X2 is an average.