Dry April and May hurt Western Colorado runoff forecasts
After a winter of near-average snowfall, Mother Nature put the brakes on Western Colorado’s snowpack beginning in mid-March. As a result, the snowpack withered prematurely and West Slope runoff has suffered, a fact compounded by the lack of subsequent spring moisture. The Gunnison River peaked in mid-May and the Colorado River peak is expected this week.
According to the Colorado River District, Western Colorado’s hot and dry summer and fall of 2019 set a poor stage for whatever snow was to come, especially in the Gunnison and San Juan basins. Dry soils absorb snowmelt before streams benefit. Lack of precipitation and high winds at the end of this past winter further decimated the conversion of snow to water supply.
“We are now in year 20 of an extended dry period that we should start accepting as the new normal,” said Andy Mueller, general manager of the Colorado River District. “Warmer temperatures, dry soils and disappointing spring and summer moisture are defining how we look at future policies to determine how best to protect Western Colorado water security.”
The bright spots for West Slope water supply continue to be in Grand and Summit counties, where the best snowpack peaked above average in mid-April and continues to be above-average for this time of the year, feeding the Upper Colorado River reservoirs such as Dillon, Green Mountain, Wolford Mountain and Granby.
The situation is much different to the west and the south with below normal snowpack and seasonal runoff forecasts that approached half of what is normally expected in the Grand Mesa zone above the Grand Valley and lower Delta County. The same holds for the greater Gunnison, Uncompahgre and San Juan river basins.
The Western Water Assessment, based at the University of Colorado-Boulder, reported that a “very dry” April in Utah and southern Colorado spurred snowmelt, while northern Colorado benefited from near- average precipitation and near- to below-average temperatures. Summer temperatures are expected to be well above average with near-average precipitation, although important seasonal monsoonal rain activity is difficult to predict.
Western Colorado contributes about 70% of inflows into Lake Powell, where the total April to July runoff forecast has now fallen to 56% of normal at 4 million acre-feet. Contributions from the Green River through Utah and Wyoming are not anticipated to be enough to offset Western Colorado’s dryness. San Juan Basin runoff is expected to be less than 50% of normal.
The accumulation of snowfall and associated runoff records over time inform water planners about drought or wet trends. Unfortunately, with fifty-six percent of normal runoff into Powell, the drought that started in 2000 continues through 2020. Lake Powell was last full in 1999. It’s just about half full currently.
Here are some reservoir outlooks throughout the Colorado River District:
− On the Colorado mainstem, Granby and Green Mountain reservoirs are expected to fill. Wolford Mountain, owned and operated by the Colorado River District, is already full.
− Ruedi Reservoir is projected to fill.
− Elkhead Reservoir and Stagecoach Reservoir in the Yampa Basin will fill.
− Blue Mesa Reservoir in the Gunnison Basin will hit 75% full due to inflows that are 54% of normal.
− Ridgway Reservoir and Taylor Park reservoirs will reach 90% capacity, with forecasted seasonal inflows of 54% and 70%, respectively.
River peaks are another data point of interest to many:
− The Gunnison River peaked on May 19 at about 5,000 cubic feet a second at the Whitewater gauge near Grand Junction.
− The Colorado River will peak this week at Cameo at about 12,900 cubic feet a second, with flows aided by upstream reservoir releases to support endangered fish habitat.
− The Yampa River near Deer lodge Park may already have seen its peak at about 13,400 cubic feet a second in early May.