Images

Geologist Michael Frothingham observes strongly deformed rocks of a Proterozoic shear zone in the Medicine Bow Mountains (Wyoming, USA).

The California Aqueduct flowing near Huron, CA in the San Joaquin Valley.  This spot is near one of several USGS land subsidence monitoring stations in the San Joaquin Valley.

National Geodetic Survey vertical control bench mark W990 CADWR in Merced County, California. W 990 CADWR is on the Mariposa Bypass Bridge on Washington Rd. This is one of several bench mark locations used to help measure the largest recent subsidence in the area using repeat surveys.

National Geodetic Survey vertical control bench mark H1235 RESET in Merced County, California. H 1235 RESET is in the median of State Highway 15. This is one of several bench mark locations used to help measure the largest recent subsidence in the area using repeat surveys.

Land subsidence contours showing vertical changes in land surface in the central San Joaquin Valley area, California, during January 8, 2008-January 13, 2010. The top graph illustrates elevation changes computed from repeat geodetic surveys along Highway 152 for 1972-2004.

Map showing estimated regions of subsidence derived from interferograms for 2008–2010 and selected surface-water conveyance infrastructure in the San Joaquin Valley area of the Central Valley, California.

Reduced surface-water availability (associated with droughts) during 1976-77, 1986-92, 2007-09, and 2012-current caused groundwater-pumping increases, water-level declines to near or beyond historic lows, and renewed aquifer compaction.

Post-subsidence diagram showing the relation between surface and subsurface processes with linkage to land subsidence. The linkages to subsidence deformation are briefly summarized for surface-water, landscape and groundwater flow processes.

Pre-subsidence diagram showing the relation between surface and subsurface processes with linkage to land subsidence. The linkages to subsidence deformation are briefly summarized for surface-water, landscape and groundwater flow processes.

The dominant cause of land subsidence in the Sacramento-San Joaquin Delta is decomposition of organic carbon in the peat soils. Under natural waterlogged conditions, the soil was anaerobic (oxygen-poor), and organic carbon accumulated faster than it could decompose. Drainage of peat soils for agriculture led to aerobic (oxygen-rich) conditions.

Reduced surface-water availability (associated with droughts) during 1976-77, 1986-92, 2007-09, and 2012-2016 caused groundwater-pumping increases, water-level declines to near or beyond historic lows, and renewed aquifer compaction.

Extensometers are used in land subsidence studies to measure the compaction and expansion of the aquifer system to some depth. This pipe borehole dual-stage extensometer was built in 2008 in San Lorenzo, CA and measures compaction from 10 to about 300 meters. The illustration highlights the features that can be seen in the photograph.

The extensive withdrawal of groundwater from the unconsolidated deposits of the San Joaquin Valley has caused widespread land subsidence—locally exceeding 8.5 meters (m) between 1926 and 1970 (Poland and others, 1975; fig. 2), and reaching 9 m by 1981 (Ireland, 1986).

Fissure near Lucerne Lake along State Route 247 (visible in background), Mojave Desert, California. The localized subsidence in five areas near dry lake beds was caused by declining water levels in fine-grained (clay and silt) sediments. In the Mojave River and Morongo Groundwater Basins (fig.

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.

Fine-grained sediments (clays and silts) within an aquifer system are the main culprits in land subsidence due to groundwater pumping. Fine-grained sediments are special because they are composed of platy grains. When fine-grained sediments are originally deposited, they tend to be deposited in random orientations.

Approximate point of maximum subsidence in the San Joaquin Valley, California. Land surface subsided ~9 m from 1925 to 1977 due to aquifer-system compaction. Signs on the telephone pole indicate the former elevations of the land surface in 1925 and 1955.

Photograph by Richard Ireland, U.S. Geological Survey