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The Sacramento-San Joaquin Delta Land-Subsidence Monitoring Network

Map of the Bay-Delta shaded by land subsidence (in 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.

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.

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.

Publications

Delta Subsidence in California: The Sinking Heart of the State
USGS Fact Sheet 005-00

Subsidence and carbon fluxes in the Sacramento/San Joaquin Delta, California
USGS Fact Sheet 049-94

Evaluation of selected data to assess the causes of subsidence in the Sacramento-San Joaquin Delta, California
USGS Open-File Report 91-193

Land subsidence in the Sacramento-San Joaquin Delta, California
USGS Open-File Report 91-452

USGS Science at Work in the San Francisco Bay and Sacramento-San Joaquin Delta Estuary
USGS Fact Sheet 2013-3037


Contacts

Michelle Sneed
Hydrologist
U.S. Geological Survey
Western Remote Sensing and Visualization Center
6000 J Street, Placer Hall, CSUS
Sacramento, CA 95819
916.278.3119
micsneed@usgs.gov

Justin Brandt
Geophysicist
U.S. Geological Survey
Western Remote Sensing and Visualization Center
6000 J Street, Placer Hall, CSUS
Sacramento, CA 95819
916.278.3159
jbrandt@usgs.gov