CORRELATING FINE-GRAINED DEPOSITS AND LAND SUBSIDENCE IN THE SAN JOAQUIN VALLEY, CALIFORNIA

Extensive groundwater withdrawal in the San Joaquin Valley from 1926 to 1970 caused widespread aquifer-system compaction and resultant land subsidence. This subsidence was largely arrested by the importation of surface water through Federal and State Water Projects; however, both expanded agricultural land use and reduced availability of surface water have caused compaction to occur in the northwestern and central San Joaquin Valley. The resulting differential subsidence has reduced freeboard and flow capacity of the Delta-Mendota Canal, the Eastside Bypass, and other canals that deliver irrigation water and transport floodwater.

The location and magnitude of subsidence during 2006–12 were determined using Interferometric Synthetic Aperture Radar (InSAR), Global Positioning System (GPS), and other land-surveying techniques. Results of the InSAR analysis indicate that a 3,200 square-kilometer area was affected by at least 20 millimeters (mm) of subsidence during 2008–10. Within that area, InSAR analysis also indicates a localized maximum subsidence of at least 540 mm. Additional land-survey data indicate these high rates continued through 2012.

The Chowchilla River and Fresno River alluvial-fan deposits, which were derived from non-glaciated Sierra Nevada terrain and contain a higher fraction of fine-grained sediments than the adjacent fan deposits derived from glaciated terrain, underlie the maximum impacted area. Analysis of data from extensometers, continuous GPS stations, and groundwater levels indicates that nearly all of the aquifer-system compaction occurred below the top of the Corcoran Clay (CC). Results from a 1-D compaction model, constrained by lithological descriptions and consolidation tests of continuous core, indicate that less than 3 percent of the total compaction occurred within the CC, which was unexpectedly small. Accurate characterization of the lateral and vertical extents, mechanical properties, and flow regimes of fine-grained deposits is critical for development and calibration of numerical models and development of realistic simulated predictions of subsidence needed by agencies charged with maintaining the integrity of water-conveyance infrastructure in subsiding areas.