Even with a 3 percentage-point dip in snowpack across Montana during February, stream flows this spring and summer won’t be too far from average.
The Natural Resources Conservation Service estimates stream flows ranging from 82 percent of normal on the lower Yellowstone to 101 percent of average on the St. Mary in northwestern Montana. The lower Yellowstone is the stretch of river from Custer to its confluence with the Missouri. That northeastward stretch of the river is mostly fed by Wyoming mountains.
The upper Yellowstone — the stretch between Yellowstone Park and Custer — is forecast to flow at 86 percent of average.
Statewide, the average forecast is for 90 percent of normal. Those with the best chance for good water are the Flathead and Milk rivers, with a forecast of 99 percent of average, and the Gallatin, at 97 percent. The Smith-Judith-Musselshell basins are forecast at 91 percent of average, and the Missouri mainstem forecast is for 90 percent of average.
Near the beginning of each winter and spring month, NRCS issues forecasts for probable stream flows for April 1 to July 31 based on snowpack levels.
Statewide, snowpack that feeds Montana rivers and streams is about 93 percent of average, despite declining slightly for the second month in a row, said NRCS water supply specialist Brian Domonkos. He said the pack is equivalent to 85 percent of the snowpack at this time a year ago. That means stream flow forecasts are also lower than last year.
“Near normal snowpack is a good sign this time of year because at this point we are typically 80 percent complete with the normal snow accumulation season,” he said.
Snowpack this year ranges from 85 percent in the Bitterroot Basin to 119 percent in the Milk River Basin. The upper Yellowstone is at 92 percent, while the lower Yellowstone is at 89 percent.
Domonkos said that about this time last year, the lower Yellowstone Basin saw an abrupt change in weather pattern that took it from well-above normal snowpack in late February to melting in mid-March, a month and a half early.
“Two main factors contributed to the early deficient runoff — record high temperatures and the strong warm winds that descended over the Bighorn Mountains,” Domonkos said. “Thankfully, this same weather pattern does not seem to be setting up this year.”
Meanwhile, the Bureau of Reclamation issued its reservoir runoff forecast for April to July in the Bighorn Basin. Bighorn Lake, which straddles the Montana-Wyoming line, will receive an estimated 63 percent of its 30-year-average April-through-July runoff, Reclamation predicts. Buffalo Bill Reservoir near Cody could receive 82 percent, while Wind River could get 87 percent. Bull Lake reservoir will see about 89 percent of average runoff; Boysen Reservoir, 71 percent.