Colorado River Compact
The foundational agreement dividing Colorado River water between the Upper Basin (Colorado, New Mexico, Utah, Wyoming) and Lower Basin (Arizona, California, Nevada). Allocates 7.5 million acre-feet per year to each basin based on flow estimates that have proven to be significantly too high.
Why this matters
The Compact's overallocation is the root cause of the current crisis. Arizona's share was further defined by the 1944 Arizona v. California Supreme Court decision, but the total allocation still exceeds actual river flows by an estimated 20%.