The river that forms part of the border between Arizona and California is the Colorado River.

The Colorado River is one of the major rivers in the southwestern United States, flowing through seven U.S. states, including Arizona and California. It plays a crucial role in the water supply for both states, providing irrigation, drinking water, and recreational opportunities. The river forms a natural boundary between Arizona and California, particularly along the southern edge of Arizona, before it continues its journey toward the Gulf of California in Mexico. It is also famous for the Grand Canyon, through which it flows.