USGS will track cause and extent of ground sinking near California Aqueduct

Historically, extensive pumping of groundwater from the San Joaquin Valley aquifer system caused groundwater levels to decline, resulting in as much as 28 feet of land subsidence. The importation of surface water to the San Joaquin Valley in the 1970s reduced demand for groundwater, resulting in a recovery of groundwater levels and a reduced rate of land subsidence.

Groundwater pumping in the Valley has increased in recent years as drought and fish-protection measures have curtailed surface-water deliveries.

for more information http://www.usgs.gov/newsroom/article.asp?ID=2300