As early as 2020, hydrologists forecast that the level of Lake Mead, the largest reservoir on the Colorado River, could drop low enough to trigger the first water shortages in its downstream states of Arizona, Nevada and California. The three states in the river’s Lower Basin have long feared shortages. But the continued decline of Lake Mead reflects a reality they can no longer ignore: Demand for the river’s water, which supports 40 million people from Wyoming to California, has long outpaced the supply.