The federal government on Monday declared a first shortage on the Colorado River, announcing mandatory water cutbacks next year for Arizona, Nevada and Mexico. California will not be immediately affected, but U.S. Bureau of Reclamation officials warned that more cuts would likely be necessary.

The river supplies drinking water and irrigation for 40 million people. The declaration of a shortage was triggered by the spiraling decline of Lake Mead, which stores water used by California, Arizona, Nevada and Mexico.