The Sacramento-San Joaquin Delta forms where the two rivers meet on the western edge of the Central Valley. In its natural state it was a vast freshwater marsh of channels and sloughs around low islands of peat and tule.
Most of it has been converted to farmland behind levees, so much of it now lies below sea level. Much of the water supply for Central and Southern California is drawn from it, and it was designated a National Heritage Area on March 12, 2019.