Streams in the southern part are intermittent or ephemeral except for short reaches of the San Pedro River. Some reaches of the upper Santa Cruz, lower Salt, and middle Gila Rivers have perennial flow sustained by treated sewage effluent. Ground water flows toward and along the axis of the Gila River. Within individual basins, water in the basin-fill deposits moves from the mountain fronts toward the basin axis, where it discharges at land surface, or mixes with existing underflow. Ground water near the perennial streams typically is close to the land surface, but in other areas it can be more than 800 feet deep.
Ground water is the main source of water for much of the study unit.
Water use in the area is dominated by agriculture. As the population increases, however, withdrawals for domestic, commercial and industrial use increases. Water levels have declined as much as 500 feet in some areas, causing associated subsidence and earth fissures. The staff of the CAZB study unit are presently in the low-level assessment activities. Interpretive reports from the Central Arizona Basins Study Unit are being published, and are available through the Arizona District office, and at the CAZB Publications page .