Scientists have found dramatically declining snowpack across the American West over the past six decades that will likely cause water shortages in the region that cannot be managed by building new reservoirs, according to a study published Friday.

The study led by scientists from Oregon State University and the University of California, Los Angeles found drops in snow measurements at more than 90 percent of regional snow monitoring sites that have consistently tracked snow levels since 1955, said Philip Mote, director of the Oregon Climate Change Research Institute at Oregon State University.

Study authors also used modeling to show the average snowpack in the region dropped between 15 and 30 percent in a little more than a century, he said, and that modeling paralleled the actual findings based on existing measurements.

That means the region’s average snowpack has lost the equivalent volume of water that it would take to fill Lake Mead, the West’s largest man-made reservoir, Mote said.

The study appeared in NPJ Climate and Atmospheric Science and was a follow-up study to one completed in 2005. This analysis found the loss of snowpack has accelerated, Mote said.