Alluvial fans and near-surface subsidence in western Fresno County, California

Abstract

Subsidence of the land surface occurs in three extensive areas in the San Joaquin Valley and in the peat lands of the Sacramento-San Joaquin Delta at the north end of the valley. Subsidence hi the three valley areas is related mainly to progressive lowering of the artesian head in confined aquifers. In the Los Banos-Kettleman City area, largely in western Fresno County, more than 1,100 square miles has subsided at least 1 foot, and maximum subsidence in 1960 was about 22 feet. The maximum yearly rate of subsidence is about 1 foot. Locally, on the western and southern flanks of the valley, subsidence also has followed application of irrigation water to certain deposits of subnormal moisture content above the water table. This near-surface subsidence, which exceeds 10 feet in places, is superposed on the subsidence due to compaction of the aquifer system beneath.