Declining snow cover is playing a key role in decreasing the flow of the Colorado River, “the lifeblood of the Southwest,” by enabling increased evaporation. As the warming continues, increasingly severe water shortages are expected. The Colorado River Basin is the source of water for 40 million people in seven states of the U.S. Southwest and two states in Mexico. Its water irrigates over 5 million acres of cropland and supports economic activity exceeding one trillion dollars annually. Users have claims on every drop of water, so there is no slack in the system. This study aims to provide activists and policymakers with data to make smart decisions about water sustainability.