Along with sandhill cranes and antelopes, snow-weary mountain dwellers often migrate en masse to warmer spots in the spring, heading to the Desert Southwest to take a break from skiing. But in the past few years, the desert has come to some ski slopes long before those first snowbirds hit the road, in the form of orange desert dust that coats high country peaks with an eerie tint. The dust arrives on strong southwest winds preceding spring snowstorms.
The San Juan Mountains often feel the brunt of the dust events, but a recent surge of desert air brought a thick layer as far north as Summit County at the end of March. If you’ve been skiing in the high country lately and noticed the pinkish snow, no need to check your goggles. It’s red-rock dust from your favorite mountain bike trail in Moab, and the strongest storms can drop up to 419 pounds of dust per acre atop the mountain snow.
The darker-colored snow melts faster — much faster — than an unsullied surface. In recent years, peak runoff has come up to six weeks earlier than average, causing headaches for reservoir operators and ranchers trying to time irrigation of their fields.
The Western Water Assessment project at the University of Colorado has acknowledged the dust storms as a serious problem, explaining that water managers have made “high errors” in forecasting peak and daily streamflows — no small matter in a state where every drop of water is counted six times.
The issue was elevated recently as part of a draft Colorado Climate Change Report released last month for public comment. In the draft report, scientists said that global warming will make more dust available for transport and intensify the melting effects of dust on snow. The Colorado Water Conservation Board, along with scientists and other stakeholders, is currently updating the 2008 version, posted at this CWCB website: http://cwcb.state.co.us/environment/climatechange/Pages/main.aspx
Last year brought record amounts of dust to Colorado. A single 16-hour dust storm on April 8, 2013 dropped more dust on the San Juans than the annual total from any previous winter since the start of detailed measurements, says Chris Landry, director of the Center for Snow and Avalanche Studies in Silverton, which tracks the dust-on-snow events via a statewide network of observation sites.
The April 8 storm deposited about 419 pounds of dust per acre, or about 47 grams per square meter, Landry says, explaining that the melt-out equation also has to include year-to-year weather variations.
For example, 2011 saw substantial dust deposition on the snowpack, but a cold and snowy spring delayed the snowmelt to a record late date. A year later, the opposite: Heavy dust on a low snowpack and dry weather starting in March resulted in one of the earliest melt-outs on record.
Real-time observations of dust-on-snow events just started recently, but scientists have other ways to track dust deposition back through the ages. Long sediment cores from alpine lakes with distinct annual layers show that dust in the mountains didn’t increase during known historic megadroughts in the Southwest.
But dust did increase starting in the mid-1800s, when settlement and grazing started in the Southwest. The findings suggest that human disturbance to desert soils are driving the increase. The depositions decreased in the late 1800s then leveled off at about five times the natural background levels due to continued disturbance.
The most recent spike starting in the late 1990s appears to be due to increasing aridity in the Four Corners source area and increasing human disturbance of the soils.
Physical and biogenic soil crusts make the deserts naturally resistant to wind erosion — but only if they are left in place. The crusts are easily disturbed by grazing, oil and gas exploration and drilling, agriculture, and off road vehicle use. Once disturbed, soil particles can be picked up by strong winds and transported hundreds of miles from the source.
Globally, dust transport is an important part of nutrient cycles. Dust blowing off the Sahara, for example, falls into the Caribbean, where it provides nutrients for plankton.