The water level at Lake Mead, the largest reservoir in the country, is the lowest it's ever been.

The man-made lake, which straddles the Nevada-Arizona border and provides water to Arizona, California, Nevada and Mexico, is now only about 37 percent full, and the water level is expected to keep dropping over the next month or so.

If the water level at Lake Mead doesn't rise above a certain threshold again by the end of the year, it would trigger a water shortage declaration that could have major consequences for parts of the West.

Here & Now's Meghna Chakrabarti talks with Dave White at Arizona State University about why the lake is so low, and what happens if the level falls low enough to trigger mandatory water restrictions.

Interview Highlights: Dave White

On Lake Mead’s regional significance

"Lake Mead along with Lake Powell, the two major storage reservoirs on the Colorado River system, are absolutely essential to the social, economic and environmental vitality for the entire western United States. Together, the reservoirs and the Colorado river system provide at least part of the drinking water supply to nearly 40 million people in the western United States, produce millions of megawatt hours of energy production, provide habitat for threatened and endangered species and produce a tremendous agricultural crop particularly during the winter months."

On reasons for Lake Mead’s low water level

"Major contributing factors right now are a significant drought that has lasted more than a decade and a half in the western United States. Your listeners may be aware of the very acute and severe drought that California has faced over the last four years. Well, the region at large has been in and out of drought conditions for the last 16 years. The level of the reservoirs have been declining over that time.

The levels may also be declining because of the increased demand and the deficit between our annual demands and our available supplies. And finally we have the signal of climate change which is causing increased temperature and decreased surface water supply into the system. So, these three factors are the explanation for the lake levels at this time."