Low Streamflows in Northwest Montana Expected Despite Delayed Runoff

Despite the springtime moisture and cool temperatures in northwest Montana that helped bolster the snowpack and delay runoff this year, hydrologists still expect below-average streamflows through July following a dry winter that led to shallow snowpack levels in the Flathead.

The Flathead River Basin is expected to be about 80% to 90% of normal total runoff volume through July while the Sun-Teton-Marias River Basin is expected to be at about 16% of normal, according to the June 1 Water Supply Outlook Report.

Eric Larson, a hydrologist with the USDA Natural Resource Conservation Service (NRCS), says the precipitation helped streamflows in the short-term, but it wasn’t enough to compensate for the weak snowpack.

“But it certainly helps,” Larson said. “That’s the story of the last month.”

The Swan, Flathead and Whitefish mountain ranges saw significant spring storms that brought more than a foot of precipitation to some areas in May. For example, the Noisy Basin SNOTEL station in the Swan Range saw 6 inches of snow water equivalent (SWE) and around 3 feet of snow during the first week in May. The snow depth as of June 10 was 52 inches.

Over Memorial Day weekend, 3 feet of snow fell at high elevations in Glacier National Park followed by cool temperatures that has posed challenges for the Going-to-the-Sun Road plowing crew as they work to open the road.

According to plow crews, the snowpack in Glacier’s high country was the deepest it had been in the last 13 years.

“Springtime can be a volatile time of year in terms of weather, and we saw some big changes,” Larson said. “April, May and June precipitation builds on that snowpack and it delayed the snowmelt.”

The Tongue and Powder River Basins in southeast Montana received the most precipitation this spring and boosted the streamflow volume forecast from around 70% of normal to 130% through July.

SWE levels in the Flathead Basin are at 78% of normal based on the median percentage between 1991 and 2020, as of June 10, while the Kootenai Basin levels are at 87% and the Sun-Teton-Marias Basin levels are at 0%.

Larson said all of the weather stations on the east side of the Continental Divide on the Rocky Mountain Front are melted out, including Badger Pass, which is the highest elevation at 6,900 feet.

“The water supply forecast for that area is pretty grim this year,” Larson said.

Other areas of concern, according to Larson, are the Bitterroot, which has a SWE level at 4% of normal, the Smith-Judith-Musselshell basin in the central part of the state at 34%, and the Upper Clark Fork at 52%.

“Below-normal snowpack peak levels this season will likely have an impact on streamflow later this summer,” Larson said. “From a water-supply perspective, above-normal precipitation during the summer is almost always welcome and slower than normal melting of the remaining high-elevation snowpack would also help sustain closer to normal streamflows later in the summer.”