Rapid runoff

  • Dust events show up as bands of red snow in this CODOS (Colorado Dust-on-Snow) study plot image from early May.

    Center for Snow & Avalanche Studies, Silverton, Colorado
 

On April 1, it looked like this would be a banner water year for Colorado's San Luis Valley, which receives just six to eight inches of precipitation annually and relies on snowmelt to fill streams and irrigate crops. Heavy spring storms had bumped the snowpack in the surrounding mountains to 113 percent of the historic average. But just a month later, much of that snowpack had vanished, and by June 1 it was all but gone, says Steve Vandiver, general manager of the Rio Grande Water Conservation District. "There's nothing left to melt," he says. "We're done."

Vandiver, who has been a water manager in the valley for four decades, says a rapid, early runoff is becoming the new norm. Local creeks and rivers peaked in late April, more than a month earlier than usual, he says, and then dropped sharply to near base flows by early June. Last year's runoff was similar -- peak flows in early May, then an abrupt decline. "It's a freefall situation, not a nice smooth descent," he says.

What is causing the change? One possibility is the large amount of dust in the snowpack. Nine major dust storms this winter and spring left significant layers of dirt in much of the state's snow. Like dark clothing, the dark particles absorb sunlight and heat up, causing the snow to melt rapidly.

Satellite images show that the dust storms originate in northern New Mexico and Arizona. Vandiver got to see one of them up close as he drove in the desert on April 5th: "For two hours I couldn't see more than a quarter mile," he says. "There was so much sand and dirt that I thought my truck engine would be ruined."

With dust events on the rise --last year saw a record 12 storms -- water managers in Colorado are increasingly looking at land-use conditions on the Colorado Plateau. Says Frank Kugel, general manager of the Upper Gunnison River Water Conservancy District, "We're eager to get data about the generators of this dust, and to see land-use regulations and development restrictions put in place that will reduce it."