This complex, interconnected aquifer system is, in many areas, in hydrologic connection with the surface water system. The aquifer system is generally comprised of a shallow unconfined aquifer and a deeper, confined aquifer. The uppermost water-saturated layer of sand and gravel, down to a depth of about 100 feet across most of the valley, is the unconfined aquifer. Below the unconfined in the central part of the valley are a number of clay layers that serve to separate, although not totally disconnect, the unconfined aquifer from deeper water-bearing layers of sand, gravel, and fractured volcanic rocks. The deeper layers, of which there are many, together make up the confined aquifer because of the overlying and confining clays.
Water flows from many wells completed in the confined aquifer due to natural artesian pressure.
The volume of storage in the aquifer system of the valley is significant. However, not all of this water is recoverable, and in some locations and at selected depths the quality may limit its uses. The unconfined aquifer functions similar to a surface reservoir with a pattern of rising levels in the spring and early summer caused by recharge from streams, canals, and early irrigation season return flows. These increases are followed by a decline as the streamflow decreases and ground water is pumped largely for agricultural purposes. The primary crops are potatoes, carrots, small grains, and alfalfa.