Two major California canals – the California Aqueduct and the Delta-Mendota Canal – have been significantly impacted by subsidence which the state says is caused by groundwater pumping in the Central Valley. Land subsidence happens, according to the U.S. Geological Survey, when large amounts of groundwater have been withdrawn from certain kinds of rocks, such as fine-grained sediments. The water is partially responsible for holding the ground up. With the absence of that water the ground sinks. One of the concerns of this phenomenon is that once an aquifer collapses in upon itself it can no longer refill.