California's Central Valley
The Central Valley: Delta & Eastside Streams
The Delta estuary covers about 1,000 square miles, has 1,100 miles of rivers and 1,600 miles of levees.
Sacramento, San Joaquin, Cosumnes, Mokelumne, and Calaveras Rivers
The "Delta" is an estuary and is actually the reverse of a classic delta, because multiple rivers come together rather than one river expanding in width outside of it's channel.
The landuse in the Delta is predominantly agriculture. The Delta includes about 55 islands or tracts that are imperfectly protected from flooding by over 1,000 miles of levees. Many of the islands in the central Delta are 10 to nearly 25 feet below sea level because of land subsidence associated with drainage for agriculture. There are also numerous smaller, unleveed islands that remain near sea level (Galloway and others, 1999).
The Delta is a major source of fresh water for domestic, industrial, and agricultural uses as well as an important habitat to 750 animal and plant species. It also supports species listed as threatened or endangered: Delta smelt, Chinook salmon, and Steelhead.
About 83 percent of water exported from the Delta is used for agriculture. The remainder is used for urban and environmental purposes in Central and Southern California (California Department of Water Resources, 1998).
In the late 1800s, large-scale agricultural development in the Delta required levee building to prevent historically frequent flooding on the low-lying Delta islands. The levees now protect farmlands that have sunk (or subsided) as much as 22 ft below sea level. An extensive network of drainage ditches prevents these subsided islands from internal flooding. Accumulated drainage is pumped through or over the levees into stream channels.