Hydrologists: Spring runoff may run deep
Most anglers probably weren’t too concerned when the Pineapple Express highballed through Colorado last December.
That super-moist storm system, named because it originates in the South Pacific, lived up to its reputation, dumping a load of snow across much of the Colorado River basin.
This spring and summer, however, the hangover from that white Christmas may be felt by anglers, irrigators and hydropower generators.
At the recent Aspinall Unit Operations meeting hosted by the U.S. Bureau of Reclamation, hydrologists said this year’s runoff into Blue Mesa Reservoir may reach nearly double what the 29-mile long reservoir saw last year.
A handout showing basin-wide snowpacks in December was bright blue, signifying snow depths across Colorado, Utah and Wyoming reaching at least 150 percent of average.
By January, the maps were red, as snow depths dropped to a third of December’s.
“Those two storm (cycles) in the second half of the month made December for us,” said Dan Crabtree, lead hydrologist for the Bureau of Reclamation in Grand Junction. “Without them, we would have had a whole lot different year because in January nothing much happened.”
But before you start pairing up all your animals, remember it’s only a forecast and the key word is “may.”
“Remember that last year the (April) forecast almost had us spilling Crystal (Reservoir) but then the next forecast went right back down,” Crabtree said. “We rely on information from the forecasting center and when that forecast changes, our operations change with it.”
The three dams in the Aspinall Unit — Blue Mesa, Morrow Point and Crystal — not only hold water for irrigation and recreation but also for hydropower and ecological concerns through the Black Canyon of the Gunnison National Park.
You might not think much about a dam three hours away and well upstream of the Grand Valley, but what happens to the Gunnison River affects most of the population of the Western Slope.
And it means a lot of people were paying attention when the April forecast hinted runoff into Blue Mesa this year might reach 800,000 acre feet, not quite twice what the reservoir received last spring.
Too much water in the upper Gunnison means higher flows downstream.
Anglers get stranded, habitat gets inundated, water for making electricity slips away over the dam, boaters get dumped, and the city of Delta starts lifting its hem as the water rises higher and higher.
But a forecast is just that, an educated guess at deciphering what enigmatic Mother Nature has in store.
Last April, the Blue Mesa runoff forecast fluctuated from 560,000 acre feet on April 1 to 530,000 on April 15 and back up to 560,000 on May 1.
But when the final numbers were crunched, actual runoff into Blue Mesa was only 494,000 acre feet, 69 percent of the 30-year (1971–2000) average of 720,00 acre feet.
Similarly, this year’s April 1 forecast of 800,000 acre feet is sure to change by the time the water rises.
“My guess is maybe the runoff will be more similar to last year when we were down to about 70 percent of average,” offered Eric Knight, a bureau hydrologist. “It depends on what our snowpack does between now and then.”
The bureau monitors runoff and adjusts its operations through nearly daily conversations with the Colorado River Basin Forecast Center in Salt Lake City.
One more thing the snowpack might do is disappear early.
Recent April forecasts have projected high runoffs only to have them disappear when the ground soaks up all the melt, or hot winds eat the snow or dust on the snow hurries the melt.
Or maybe it’s global warming.
There are indications the runoff is trending a week or two earlier over the past four or five years, said forecasters from the Forecast Center in Salt Lake City.
“In general there is some truth to that,” said Brent Bernard, hydrological forecaster with the Forecast Center. “Our data looks at climate change studies and how that affects snow melt. In several of the last 10 years, the melt has been week to two weeks earlier.”
But Bernard emphasized there is no indication of a long-term change in snowmelt over the 30–50 year time period the climate studies are based on.
“It’s based on averages, which isn’t reality but rather a midpoint on the line of data,” he said.
Something more immediate that affects runoff is the condition of the soil going into the snow year.
“If you have a dry fall or go into winter with below average soil moisture, it takes a while to get that saturated,” Bernard said. “But a couple of years like we’ve had lately with late springs, cold temperatures and high soil moisture content, when runoff starts the system quickly gets overloaded with water.”
An example of that happened last week, when a storm dropped seven-tenths of an inch of water on the upper Yampa Valley, an area already overloaded after receiving nearly 400 inches of snow this year.
In 18 hours between April 20–22, the Yampa jumped from 2,700 cubic feet per second (approximately 20,200 gallons per second) to about 6,100 cfs (46,000 gallons).
“The sheriff told us several roads were closed after the Yampa flooded,” Bernard said. “And it wasn’t runoff. The ground is so saturated there isn’t anywhere for the water to go but into the rivers.”
Which makes forecasts of high water this spring even more intriguing.
“It’s going to be an interesting water year,” Bernard agreed.