Below Normal Streamflows Forecast in Northwest Montana Amid Early Peak Runoff
Hydrologists say rivers across the state have already hit peak flows while snow water equivalent levels in the Flathead River basin are slightly below normal at 85%
By Maggie Dresser
An early storm last month helped boost the snowpack in northwest Montana, bringing the Flathead River basin’s streamflow forecast to slightly below normal from May through July, according to the USDA Natural Resources Conservation Service (NRCS) water supply outlook report.
The Flathead River basin water volume as of early May was 80% to 100% of the median snow water equivalent (SWE) after receiving 96% of normal precipitation in April. While the precipitation was above 100% in February and March, the boost did not make up for the deficit in January that left SWE levels at 44% of normal.
As of May 16, the Flathead River basin SWE levels were at 85% while the Sun-Teton-Marias River basin was at 42%. The Kootenai River basin’s levels were at 73%.
NRCS hydrologist Florence Miller says most weather stations have passed the peak SWE accumulation for the season after warm temperatures last month caused early runoff. As upper elevation SNOTEL sites continue to melt, hydrologists say much of the low elevation snow is already gone.
“This was accentuated by several April warm spells that brought periods of above normal temperatures and initiated snowmelt across many low- to mid-elevation SNOTEL sites,” Miller said in a news release.
Statewide temperatures averaged slightly above normal last month while northwest Montana saw average temperatures more than 5 degrees above normal during the last week in April.
National Weather Service hydrologist LeeAnn Allegretto, too, said in a May 14 weather briefing update that most river flows across the state have already peaked.
“So far the first few weeks of May have been well above normal temperature trends and that has combined with a well below normal precipitation trend, which has caused rapid snowmelt in the high terrain,” Allegretto said. “That has led to early peaks in river flows.”
As of May 16, the Middle Fork Flathead River’s streamflow was 9,390 cubic feet per second (cfs) while the most recent peak flows hit 12,400 cfs on May 11. The North Fork’s streamflow was 9,740 cfs, with recent peak flows at 12,800 cfs on May 13.
While this winter saw a greater snowpack statewide compared to 2024, May’s SWE levels are roughly the same as last year following the warm temperatures.
“Even though we had more snowpack to start, we have melted out so much that we’re looking very similar in a lot of the subbasins to last year at this time,” Allegretto said. “So unfortunately, we were not able to remain cool enough or wet enough to hang on to the snowpack that we had and thus we have seen a high erosion of our high terrain snowpack.”
Despite the warm temperatures, deep snowpacks still exist in the Swan Range with SWE levels at 108% in Noisy Basin, which as of May 16 had a snow depth of 85 inches at an elevation of 6,040 feet. The weather station in April saw the highest total accumulated precipitation in the state with 7.5 inches.
Due to the deep snowpack in the Swan Range, Allegretto said the South Fork Flathead River is showing strong streamflows this year, with current forecasts showing melt off in mid to late June.
Just across Flathead Lake, however, Blacktail Mountain saw the lowest precipitation in the state with 1.2 inches accumulated in April.
Hydrologists say future weather will determine this season’s runoff season. A cooler spring would facilitate a gradual runoff while additional precipitation would help improve the current streamflow conditions.
“We’ve seen our peak – we are melting out fast,” Allegretto said. “It would take a lot for us to maintain or even gain snow going into the end the latter half of May and June.”