Which water source is federally regulated in Arizona?

The Colorado River is federally regulated in Arizona due to its significance as a key water source that crosses multiple state lines and is subject to federal laws and treaties. The river is governed primarily by the Colorado River Compact, which was established in 1922 and allocates the river's water among the seven basin states. This federal oversight plays a critical role in managing water rights, usage, and conservation efforts for the river, which has a vast impact on Arizona's water supply and management practices.

In contrast, the other rivers mentioned, such as the Gila River, Pecos River, and Salt River, are primarily regulated at the state level without the same degree of federal involvement. While these rivers also play important roles in Arizona's water resources, they do not have the same federal regulatory framework as the Colorado River, which reflects the importance of the river in not just Arizona, but also nationally due to its shared nature among several states.