Subsidence in the Sacramento-San Joaquin Delta

Science Center Objects

The Sacramento-San Joaquin Delta is part of the San Francisco Estuary, home to a diverse flora and fauna, including several threatened and endangered species, has a large area of prime farmland, and serves as the hub of California's freshwater-delivery system that moves water from the wet north to the dry southern part of the State.

Map of land subsidence in the Sacramento-San Joaquin Delta, shaded by feet below sea level

A map of subsidence in the Delta based on the leveling and observations of transmission-line foundations, circa 1930s-1990s. The subsidence increases stresses on the levee system, and failure of levees would cause salt water to move further up the Delta system by disrupting favorable gradients. This would degrade the quality of water that is the heart of water supply for California. (Public domain.)

Beginning in the late 1800s, the Delta's vast historical wetlands were drained to make way for agriculture on dry "islands" surrounded by waterways and protected by 1,100 miles of levees. Exposure of previously water-logged wetland peat soils to air caused them to decompose and subside below sea level by 9 to 26 feet or more. The subsided Delta islands are perpetually at risk of flooding in the event of levee breaks or overtopping and many have flooded in the past, causing millions of dollars in damage. As subsidence progresses, the levees must be regularly maintained and periodically raised and strengthened to support the increasing stresses on their banks. Delta island flooding can also interfere with freshwater exports from the Delta.

USGS studies about subsidence in the Delta have focused on rates of subsidence, how the Delta's thick peat soils were created, and ways to mitigate or reverse peat soil degradation. For example, on deeply subsided Twitchell Island in the Delta, the USGS spearheaded the creation of an experimental wetland that, through the growth of marsh plants, "sequestered" or stored carbon, accumulated peat sediments, and reversed subsidence.