San Francisco Bay is a dynamic tidal system and a complex estuary, which is a closed embayment where fresh water and saltwater mix. Ocean tides enter and leave the Bay twice a day. Between high and low tides, changes in the surface of the water may be as much as 9 feet. The average range, 4 to 4-1/2 feet, moves 390 billion gallons of salt water through the Golden Gate. This volume between tidal elevations is called the tidal prism.
Also affecting the Bay's tidal system is the water flowing into the Bay from rivers, streams and creeks. Runoff following rainstorms and snowmelt from the Sierras, travel through either the Sacramento or San Joaquin River to their final destination in the Bay.