Glen Canyon Dam's most vital purpose is to provide storage to ensure enough water flows from the Upper Colorado River Basin to the lower, especially in drought years. The 1922 Colorado River Compact requires annual delivery of to the Lower Basin states of Arizona, California and Nevada; the 1944 treaty with Mexico obligates the U.S. to allow at least for use in the Mexican states of Baja California and Sonora. Glen Canyon Dam must supply at least of this water; the remaining comes from other tributaries of the Colorado River. The required release from Glen Canyon is averaged over a 10-year period, so releases in each year may be higher or lower depending on the amount of runoff. In wetter years, the Bureau of Reclamation may decide to release extra water from Glen Canyon Dam if the level of Lake Powell exceeds the "equalization tier", an elevation determined by the difference in storage between Lake Powell and Lake Mead.