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Increasing snowpack, rising temps, decreasing river flow

Annie Whetzel

If you've been reading the news about the Colorado River and the Roaring Fork watersheds, I'm sure you have noticed that many articles highlight the increased snowpack, and unusually heavy, water-laden snow. But recently, a few articles cover climate change and explore how rising temperatures and early snowmelt-season can result in decreased flow in the Colorado River.

There is good news and bad news in the latest information, so let's navigate these waters together.

First, the good news: There is increased water in the snowpack for this year in our area. That means, not only do we have an increase in snowpack — 55 percent more than normal, according to the latest report from the Roaring Fork Conservancy — it also means more water in the snow than usually found. The light fluffy powder that Colorado is known for hasn't been as common this year.

Because of the increased water in the snowpack, we should anticipate a good year for water. New reports show, however, that might not be the case.

In a recent accepted article in the Water Resources Research, climate models show rising temperatures and this, the researchers find, could lead to less water flow in the Colorado River. Flow is determined by the amount of water in a cubic foot rushing past per second. If there is decreased flow, that means there is less water in the system.

Bradley Udall at Colorado State University and Jonathan Overpeck at the University of Arizona, the lead researchers in the accepted article, found that based on drought data over the last few decades the Colorado River flow is expected to be below normal because of decreased precipitation, but not to the extent they observed.

Instead, when they examined the most recent drought spanning 2000-2014, the flow in the Colorado River flow decreased by 19.3 percent, and almost one-third of that decrease they found is due to rising temperature.

"In the 20th century, droughts were associated almost exclusively with a lack of precipitation. In this century, however, high temperatures alone can lead to anomalously dry conditions," write Udall and Overpeck in their report.

In the simplest terms, rising temperatures create dry air and can increase the effects of evaporation. A new study published in Nature Climate Change, however, explores how warmer temperatures cause an earlier snowmelt season, and can also decrease flow in the river.

Keith Musselman, a postdoctoral research fellow at the National Center for Atmospheric Research in Boulder and lead researcher in the slow snowmelt study, said that when the snow melts earlier in the year, it creates a slow trickle from the snowpack and rather than the spring surge rivers are accustomed to. Much of this is because, of all things, the angle of the sun in the late winter months.

"When snowmelt shifts earlier in the year, the snow is no longer melting under the high sun angles of late spring and early summer," Musselman explained. "The sun just isn't providing enough energy at that time of year to drive high snowmelt rates."

A slower snow melt rate gives the ground more of an opportunity to absorb the water, thereby decreasing the amount of runoff available to the stream. In terms of flood management, this is reasonably good news for spring flood mitigation. It is less optimal for riparian plants and animals that rely on the surges of spring runoff and flood plains for a healthy ecosystem.

"Higher flows are critical to maintaining healthy river ecosystems," Liza Mitchell, the Outreach and Education coordinator for the Roaring Fork Conservancy explains, "peak flows reconnect the channel with the floodplain, redistributing nutrients and saturating the riparian zone 'sponge,' and are also strong enough to scour the streambed and move the substrate. Scouring can prevent cobbles from becoming too embedded, and opens up the interstitial spaces between the rocks that provide habitat for aquatic macroinvertebrates – the keystone to the stream food web."

Slow snowmelt and runoff and increased absorption by plants also means less water in the river for human use and consumption.

Despite the increased amount of snowpack this season, we will need to stay vigilant and see what happens with river flow as we enter melting season. Enjoy the existing snowpack and we'll see what springtime temperatures hold.

Annie Whetzel is community outreach coordinator for The Middle Colorado Watershed Council, which works to evaluate, protect and enhance that watershed through the cooperative effort of stakeholders. To learn more, go to http://www.midcowatershed.org or find the MCWC on Facebook at http://facebook.com/midcowatershed.