The decades-old agreements that outline water rights to the Colorado River Basin are leading to an impasse on an issue affecting millions of people in the American Southwest.

On Jan. 31, the seven states that draw water from the basin had to come up with a plan to voluntarily cut back on using water from the basin. Six states — Arizona, Colorado, Nevada, New Mexico, Utah and Wyoming — agreed on one proposal. But California, which is the state the uses the most water, rejected that plan and submitted its own.