Lee Ferry on the Arizona-Utah border is a key dividing point between the Colorado River’s Upper and Lower basins.
This split is important when it comes to determining how much water will be delivered from the Upper Basin to the Lower Basin [for a description of the Upper and Lower basins visit the Colorado River page].
Under the Colorado River 4.4. Plan—which apportions the river among seven states , the Upper Basin is committed to deliver 75 million acre-feet to the Lower Basin every 10-year period. The deliveries occur at Lee Ferry.
After the Lower Basin states (Arizona, California and Nevada) and Mexico have received their respective apportionments, the Upper Basin states (Colorado, New Mexico, Utah and Wyoming) retain the remaining water supply. The Upper Basin may not “unreasonably” withhold water the Lower Basin needs for current beneficial uses.