Rising Temperatures and Dwindling Snowpack Are Draining Mountain Streams

Scientists are working to find out how rising temperatures and more severe droughts caused by climate change will affect the precious mountain streams of the American West, which feed into watersheds that supply freshwater for millions of people.

It turns out that how water leaves a watershed matters more than whether or not it comes in as either water or snow, according to a study published Tuesday in the journal Environmental Research Letters.

“We already know that climate change is going to affect temperatures faster in the mountains than other climate regions, so we need to get a better understanding of how these influences are going to affect water supplies in the future,” said University of Utah geologist Paul Brooks, a coauthor of the study. “We need better information so we can allocate water accurately and be prepared for what’s ahead,” he explained.

To get that information, Brooks and fellow researchers used computer models to test how snowpack, stream, groundwater, and reservoir levels in two Colorado watersheds would respond to two climate scenarios. In one, precipitation came down only as rainfall—no snow. In another, annual temperatures rose by 4 degrees Celsius.

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“We found that if all precipitation came as rainfall, the rivers and streams would be ‘flashier,’ meaning they would rise and fall faster,” Brooks said, “instead of the more steady source of water snowpack provides.”

The “no snow” scenario led to an 11-percent to 18-percent decrease in overall stream flows, while turning up the heat increased evaporation, resulting in stream levels reduced by more than 20 percent.

In the earth’s normal water cycle, a large percentage of precipitation doesn’t make it downstream. Water is lost to soil and taken up by plants, or it evaporates directly back into the atmosphere. Rising temperatures could kick this evapotranspiration into overdrive, the study found.

Such a decline might force certain areas in the West to rely on groundwater reserves an entire season earlier—beginning in summer instead of fall—as stream flows decline.

The West’s mountain rivers and streams buoy an environment that millions of birds, fish, and other wildlife rely on and an economy of millions of humans depend on as well.

The majority of that water supply comes from snow—stored on mountain peaks in the winter, melting in the spring to replenish river runs and groundwater supplies and sustaining reservoirs levels at lower elevations through the summer.

“Look at just the Colorado River basin—the states that rely on it for water supply make up the fifth-largest economy in the world,” said Brooks.

But if that cycle is broken—whether by rainfall replacing snowfall or higher temperatures increasing evaporation rates—the water competition at lower elevations such as California’s Central Valley could escalate further, he said.

“If that water isn’t in streams during the summer, when people need it, when farmers are irrigating almond trees and other crops and fish are spawning, then it puts stress on every part of the puzzle,” said Brooks. “From the natural system to day-to-day life, we’ll be competing for a smaller and smaller amount of water.”