Recently Measured Rapid Land Subsidence in Eastern San Joaquin Valley, California

Rapid land subsidence was recently measured in the Eastern San Joaquin Valley, between Merced and Fresno. This area includes the San Joaquin River and Eastside Bypass, the primary flood control channel east of the river. Global Positioning System (GPS) surveys in 2008 and 2010 measured 0.5 m of subsidence adjacent to the Eastside Bypass. Subsequent interferometric processing of synthetic aperture radar data for the same period confirmed the GPS measurements, and further revealed a larger area—100 km east-west by 30 km north-south—affected by at least 20 mm of subsidence. The maximum measured rate, about 0.26 m/yr, is among the largest rates measured in the San Joaquin Valley, and about 20 times that reported for this area during 1926–70, a period of extensive subsidence in the western and southern parts of the valley. Groundwater is the primary water source in the Eastern San Joaquin Valley, and groundwater levels have declined by more than 55 m since the late 1960s. The magnitude and extent of subsidence in the area during the past four decades is largely unknown.

Land subsidence can greatly reduce the flow capacity of rivers and water-conveyance systems, which can exacerbate flood hazards and affect the delivery of irrigation water. Subsidence-related changes in flow and streambed conditions raise concerns related to the restoration of the San Joaquin River. Monitoring networks and hydrogeologic investigations are needed to better understand and characterize the subsidence hazards. Time series of historical subsidence and groundwater levels are needed for monitoring network design. Existing and new geologic logs are needed to better characterize the depths, extents, and thicknesses of highly compressible units.  Data collected from the monitoring networks will improve simulations of groundwater flow and subsidence, and facilitate the evaluation of management strategies to mitigate subsidence hazards to infrastructure and habitat while maintaining optimal water supplies.