The surface level of Lake Mead in Nevada and Arizona has fallen to a historic low as 16 years of ongoing drought in the American Southwest continue to impact the Colorado River Basin.

Landsat imagery captures the decline of the country’s largest reservoir. In a May 1984 Landsat 5 acquisition, Lake Mead is almost full. But 32 years later, in May 2016, Landsat 8 data show the reservoir when it was 37 percent full. The drop in lake level isn’t even as apparent as it might otherwise be because of the steep topography in the region, but the surface area reduction is still quite noticeable.

Today, Lake Mead supplies water to 25 million people. Virtually all of nearby Las Vegas, NV, with its 2 million residents and 40 million tourists a year, gets its drinking water from the shrinking reservoir. Lake Mead also serves farms, tribes, and businesses in Arizona, California, Nevada, and northern Mexico.

According to the U.S. Bureau of Reclamation, Lake Mead reached a historic low in May 2016 of 1,074 feet above sea level. It has not been this low since the reservoir began filling in the 1930s.