Water flows from the bottom of Boysen Dam into the Wind River. (Angus M. Thuermer Jr/WyoFile)

The water content of Wyoming’s snowpacks dropped from 100% of median to 92% in the last two weeks, the U.S. Natural Resources Conservation Service reports.

The weekly update comes as rivers rise toward their crests, irrigators open headgates and recreationists take to lakes and rivers to row and paddle for fun and sport. The readings show ample water supplies for most of the state, underscore some areas of worry and hints of regional drought.

The Lower North Platte basin registered the highest reading at 108% of the median. Comparisons are made to the 30-year period from 1981 and 2010.

Two lower-elevation areas, the Belle Fourche and Cheyenne river basins, clock in at 0% of the historic median, a common occurrence at the end of the runoff season when the snowpack dwindles and readings fluctuate radically.

The Sweetwater and Wind river drainages also show low readings — at 22% and 61% of median respectively. Those measures appear to reflect dry times in the Wind River Range, according to experts.

The Lower Green River drainage also measures 61% of median snow-water equivalent, but an asterisk on maps and charts mark that reading as suspect. The Upper Bear River measures 57%.

Other mountain snowpacks reflect healthy readings, ranging from 103% above the Yellowstone River, 90% above the Upper Green River and 78% above the South Platte, according to a map (below) published Monday. The climate office at UW published different readings Tuesday.

Low-elevations melt out

When basin percentages are averaged statewide and weighed according to land area, the amount of water in Wyoming’s snowpacks registered 65% of average Monday, the NRCS reported. The Belle Fourche and Cheyenne basins dragged that statewide average down, said Jeff Goats, a soil scientist with the agency. He assembles the weekly summary of readings from remote stations.

The water content of snowpacks in river basins across the state on May 18, 2020, is compared by the Natural Resources Conservation Service to the median during the 1981-2010 period. (NRCS)

“They probably have melted out,” Goats said of the far-east Wyoming drainages. Tony Bergantino, interim director of the State Climate Office at the Water Resources Data System at the University of Wyoming, said those measurements are “not too meaningful.” 

“You’re dealing with a small ratio,” Bergantino said. “If reality changes by an inch, you’re looking at a major change in percentage.”

“You’ve got two basins out there both of [which] are coming in at zero,” he said. “Those are bringing the state down.”

Abnormally dry

A drought monitor map for May 12 shows abnormal dryness in north-central Wyoming, including parts of Big Horn, Washakie, Johnson, Campbell, and Crook counties and all of Sheridan County. “Abnormally dry” is the least severe measure on a five-step drought scale.

The map also shows abnormal dryness in the southwest corner of the state covering Uinta and parts of Lincoln, Sweetwater and Sublette counties. Most of the Wind River Range in Fremont and Sublette counties also is affected.

The Wyoming water supply outlook lists acre-feet of river flow expected from May through July and color-codes rivers according to their relationships to average runoff. (NOAA/NRCS)

“This is sort of a warning, a heads-up we’re starting to get down there,” Bergantino said.

Forecasters expect the New Fork, Big Sandy, Sweetwater, Little Popo Agie, Hams Fork and Bear rivers to carry runoff volumes, measured in acre-feet, that are less than 80% of normal. The Little Laramie River is expected to have more than 115% of normal volume this year, according to an outlook map compiled by the National Weather Service and posted May 8 at the UW site.

Residents can keep track of flooding potential through the National Weather Service’s Advanced Hydrologic Prediction Service — an interactive map.

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A teacup diagram of reservoirs shows the level at which they have filled along with acre-feet stored.

The Med Bow SNOTEL site at 10,500 feet registered the deepest snowpack of all sites this week, a depth of 87 inches. It sends signals from east of Saratoga in the Medicine Bow Mountains.

Angus M. Thuermer Jr. is the natural resources reporter for WyoFile. He is a veteran Wyoming reporter and editor with more than 35 years experience in Wyoming. Contact him at angus@wyofile.com or (307)...

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