Most are in the mountains and other higher elevations. Further downstream, dams and canals store and divert surface water for use in cities and farms, provide flood protection, and release regulated flows into river channels. The flows in Arizona's streams and rivers vary greatly over time, from minimum base flows in dry times to dramatically higher volumes during wet periods. Interrupted streams alternately flow in some stretches and disappear underground in others. Despite extensive networks of water control structures on the state's major rivers, flows can reach flood stage, with rampaging water eroding banks, knocking out roads and bridges, and inundating neighboring lands.
Droughts can reduce normally significant flows to a trickle.
Despite their erratic behavior, rivers are a key component of Arizona's water supply because they are renewable supplies. Watersheds are land areas that drain into a single river system. All the major watersheds in the state drain westward away from the Continental Divide and into the Colorado River. Adjudicating or settling ownership of surface water rights is a major state concern. Current efforts to adjudicate Indian and other water rights for the Gila and Little Colorado River watersheds will take many years.