Land subsidence in the San Joaquin Valley, California, as of 1980

Land subsidence due to ground-water overdraft in the San Joaquin Valley began in the mid-1920 's and continued at alarming rates until surface was imported through major canals and aqueducts in the 1950 's and late 1960's. In areas where surface water replaced withdrawal of ground-water, water levels in the confined system rose sharply and subsidence slowed. In the late 1960 's and early 1970 's water levels in wells recovered to levels of the 1940 's and 1950 's throughout most of the western and southern parts of the Valley, in response to the importation of surface water through the California aqueduct. During the 1976-77 drought data collected at water-level and extensometer sites showed the effect of heavy demand on the ground-water resevoir. With the ' water of compaction ' gone, artesian head declined 10 to 20 times as fast as during the first cycle of long-term drawdown that ended in the late 1960's. In the 1978-79 water levels recovered to or above the 1976 pre-drought levels. The report suggests continued monitoring of land subsidence in the San Joaquin Valley. (USGS)