Scientists warn there is a 50% chance that Lake Mead—which, along with Lake Powell, helps to provide water for more than 25 million people in Nevada, California and Arizona, as well as northern Mexico—will be dry by 2021.

Located along the Colorado River, and spanning the Nevada and Arizona borders, Lake Mead provides 90% of Las Vegas’ water supply.

Growing populations and natural forces, such as drought and evaporation, are beginning to threaten the lake, the West’s largest storage reservoir. Researchers also calculate that by 2014, if the regional drought continues and water usage escalates, there is a 10% chance Lake Mead will no longer supply usable water.

“We were stunned at the magnitude of the problem and how fast it was coming at us,” said marine physicist Tim Barnett. “Make no mistake, this water problem is not a scientific abstraction but rather one that will impact each and every one of us that live in the Southwest” (Associated Press).

Currently, the lake is half-full, leaving a shoreline water ring visible from high above in the sky. Researchers say if the lake drops lower than 1,000 feet, Nevada would lose access to all of its river distribution and Arizona would lose most water that flows through the Central Arizona Project Canal. Power production would cease before the lake dries up completely.

Whether Lake Mead runs dry will depend upon the natural fluctuations of the Colorado River, which feeds it. In recent years, the river’s flow has been below average. “The biggest change right now is taking more water from the bucket than we are putting into it,” Dr. Barnett said (Reuters).