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Will big snowpack bring floods to Colorado Front Range?

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Brian Calvert | May 23, 2014 05:00 AM

Dan Barber, the deputy director of Boulder County’s office of emergency management, likes to think in terms of stream flow.

At the height of the Front Range flood in September 2013, Boulder Creek was roaring through the canyon at a flow rate of 5,500 cubic feet per second. To Barber, that’s a more interesting number than the 25-year or 100-year flood that got bandied about.

With that much water moving through the mountains, sediment, trees, boards – all manner of debris – shot through the drainages of the Front Range, particularly in Boulder and Larimer counties. Some of the creeks have since been cleared out, but many of the roads that were destroyed by the flooding have only received short-term repairs.

This year’s snowpack has been a mixed bag across the West. Some places remain very dry, but in some, like northern Colorado, late season storms bolstered the snowpack. And the later the snowpack stays without melting out, the more worried water managers and other weather watchers get.

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Colorado, Rio Grande, and Arkansas River Basins Mountain Snowpack as of May 1, 2014. Map of the Natural Resources Conservation Service.

So with temperatures warming and a hard rain coming down on the plains east of Boulder on Tuesday, Barber has been thinking of stream flow a lot lately.

“It’s an anxious time right now,” he told me on Thursday morning, as the flow of Boulder Creek crept toward 500 cubic feet per second. Barker Reservoir, a catchment below the nearby town of Nederland, was likely to spill its banks in the next few days, he said, pushing that flow rate up toward 700 (the rate at which Boulder bans river tubing).

And there’s even more water up there, locked in snow. Most of the mountains of northern Colorado still have major snowpack, upwards of 150 percent of normal, and spring temperatures are warming as we slide toward summer, lessening the chance of a gradual melt-out, Matthew Kelsch, a hydrometeorologist at the University Corporation for Atmospheric Research, said. “The bad news is, we’re already in the middle of May.”

For the Front Range, that’s a problem.

No one knows how any of those streams will act when the snowpack starts to flow, Barber said.

The 2013 flood created more than 1,000 small landslides, pushing sand, gravel, trees and other woody debris into the creeks. Some streams were gouged out; others filled in. The streams have changed so much that prediction models for the Front Range drainages simply won’t work until engineers get the streams figured out again, Barber said.

Major storms east of the mountains have people watching closely, he said. Aurora got nearly an inch of rain in 15 minutes on Wednesday. “If we get an inch in 15 minutes, we’ve got major problems all over the foothills,” he said.

Streams might flood, or clog, roads – temporarily fixed after the flooding – could “melt,” and more landslides could give way, he said.

Barber, Kelsch and others are keeping a close eye on the weather. The National Weather Service is showing some chance of rain this weekend, but nothing is certain. And there's still a chance of a slow melt-out. Over in Larimer County, emergency managers are worried about a potential high-altitude rain today that could create even more problems.

To help managers as the spring progresses, Barber recommends people watch for the out of the ordinary: sagging earth, leaning trees, or streams that have stopped and could break free. People should report these things to 911, he said. “They should also get the heck out of there.”

Brian Calvert is the associate editor at High Country News.

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