This graph shows the average snow-water equivalent reading recorded across the five SNOTEL stations of the Roaring Fork basin between December 2020 and July 2021. SWE at these stations declined by roughly 10% on average during the April 1-7, 2021 heat wave, a phenomenon across the West described in a study by the Desert Research Institute.

Water managers in the Colorado River basin are gaining a better understanding that what happens in the weeks after peak snowpack — not just how much snow accumulated over the winter — can have an outsize influence on the year’s water supply.

Water year 2021 was historically bad, with an upper basin snowpack that peaked around 90% of average but translated to only 36% of average runoff into Lake Powell, according to the U.S. Bureau of Reclamation. It was the second-worst runoff on record after 2002. One of the culprits was exceptionally thirsty soils from 2020’s hot and dry summer and fall, which soaked up snowmelt before runoff made it to streams. But those dry soils are only part of the story.

McClure station

Heather Sackett/Aspen Journalism

This SNOTEL site at the top of McClure Pass is one of hundreds of remote sensing stations throughout the West that monitor weather and precipitation data in mountainous terrain. New research shows that there are many more factors relevant to predicting water supply than just how much water is in the snowpack, a metric known as snow-water equivalent (SWE), which is measured by SNOTEL sites.