The reservoir near Las Vegas has fallen to its lowest levels since Hoover Dam was built in the 1930s and is continuing to drop after years of chronic overuse and drought intensified by climate change. It now stands at just 35% of full capacity.
To try to prevent Lake Mead from falling to critically low levels, state officials from Arizona, California and Nevada announced that they've begun meeting to discuss potential additional steps, which could include more water cuts.
Federal water managers said the first shortage declaration shows how severe the drought has become and how climate change is having serious effects on the river, which provides water for about 40 million people.
"Reclamation does not take these actions lightly or do so easily. We do so because it is necessary, protecting the system and implementing the agreements we have in place," Reclamation Deputy Commissioner Camille Touton said in a news conference.