Where spring runoff has been something like average—and where it hasn’t
by Allen Best
STEAMBOAT SPRINGS, Colo. – Spring runoff of the Yampa River likely peaked on May 14 this year as it flowed through northwestern Colorado. That makes it an anomaly in the precipitation-dripping mountains of the West.
In most other locations, the peak runoff—the time when the largest volume of water in rivers occurs as winter’s snow melts—more normally occurs in early June after temperatures have finally warmed. This year looks to be more or less normal, despite a trend to earlier runoff in many locations during the last several decades.
“The Yampa did have an early runoff, and that was the result of the warm temperatures and below-average snowpack,” said Ashley Nielson, senior hydrologist with the Colorado Basin River Forecast Center in Salt Lake City, when interviewed last week by Mountain Town News. The Yampa, she noted, will probably rise again in the next week or so, if not to the same high mark.
But elsewhere, the show is occurring now. Peak runoff of the Green River was expected this week or next. The river originates in the Wind River Range of west-central Wyoming. Unlike the Yampa, that basin still has a significant snowpack. That was also reported to be the case in Jackson Hole, at the headwaters of the Snake River. The snowpack there was 181 percent of average in late May, not a record but “up there,” in the words of one water official cited by the Jackson Hole News&Guide.
Peak runoff in the upper Colorado River at its headwaters along the Continental Divide in Colorado was also expected to occur in early June.
Winter had wild swings: barren until late fall, then torrents of snow in December and January. Temperatures were unseasonably warm in February and almost hot in March. It looked like an early runoff everywhere. Then May turned cold and snowy.
What explains the Yampa’s aberrant behavior? Karl Wetlaufer, a hydrologist with the Natural Resources Conservation Service in Denver, said the peak snowpack in northwestern Colorado arrived about a month earlier than usual. That snowpack around Steamboat Springs occurred on March 12, compared a more typical April 10.
Instead of mid-May for the Yampa, he says that rafters floating through Dinosaur National Monument more often experience the highest water flows of the year in early June.
Flows in the Animas River through Silverton and Durango have had some “pretty wild swings,” Wetlaufer says.
The Snake River of Wyoming and Idaho has a very different story than the Yampa, with around 200 percent of snowpack this year. The Snake originates in Jackson Hole and picks up water from the Big Wood River, which originates in the Sawtooth Mountains above Ketchum and Sun Valley, before joining the Columbia at the Idaho-Washington border.
“My takeaway is that this year is pretty normal” in terms of timing, says Bruce Anderson, the senior hydrologist at the Northwest River Forecast Center, in Portland, Ore. It was cooler and wetter in spring, but the big story was the amount of precipitation that fell during winter. “We are hugely above normal for precipitation.”
In the Tahoe-Truckee area of California’s Sierra Nevada, the snowpack was among the deeper ones on record after three bad drought years and then a so-so winter in 2015-16. Snowfall this winter was not a record, but it was a record for total precipitation. Being somewhat lower and closer to the coast than Colorado, the Sierra Nevada gets more rain during winter. This year it got a lot of rain.
Colorado, too, had rain on snow, which is not unprecedented. But it happened frequently this winter. The result was telling for travelers on I-70 when crossing Vail Pass.
“In general, there was less snow than you would expect,” says Klaus Wolter, a research scientist at the National Oceanic and Atmospheric Administration in Boulder.
Were those rain on snow storms of this past winter a result of accumulating greenhouse gas emissions? Wolter told Mountain Town News that thinks this is “probably partially climate change.”
Wolter, whose focus is empirical climate research, using statistical methods to solve climate problems, is reluctant to pin climate change on much of what we have seen this year. True, he says, one storm during May left 42 inches of fresh snow in the foothills above Boulder, a storm unprecedented since the 1920s. As extreme as that storm was, proving causality is difficult, he says.
A scientist in Oregon also shared the difficulty of proving causality. John Stevenson of Oregon State University told the Idaho Mountain Express in Ketchum that it’s “really difficult to judge any one year” to be a result of rising global temperatures.
“That’s one of the challenges we run into in the science world where people say, ‘Oh, it’s climate change.’ We’re not at the point where we can take any one random event and say it’s climate change.”
That said, his 2015 study concluded that the point each spring when half of the water year’s streamflow had run off was occurring an average 1.9 days earlier per decade.
But more extreme events are happening with greater frequency, said Mark Davidson, director of conservation initiatives with The Nature Conservancy. He pointed out that the Big Wood River has had two 100-year floods in the last 15 years.
Temperatures in the Ketchum and Sun Valley area were 6 to 13 degrees warmer than normal for early May, producing a flood in the Big Wood River that peaked on May 8. It was regarded as the largest in 101 years of recorded history, reports the Idaho Mountain Express.
More warm weather was producing another surge in early June that threatened to surpass that peak of a month before, the Express reported last week.