Montana streamflows likely to be normal this summer after snowy April

After a winter of extreme weather swings, Montana managed to get enough mountain snow in April that most rivers should run at normal levels this summer, barring sudden drought, heat or overuse.

As of May 1, snow in most western Montana mountains matches the 30-year average, with the Bitterroot River basin coming out a little ahead and the Kootenai basin a little behind, according to the May U.S. Natural Resources and Conservation Service report.

After February’s record rash of storms and March’s record lack of moisture, April came through with above-average precipitation in much of western Montana, except along the southwest Idaho border.

“Some monitoring sites in valleys and mountains of the Bitterroot, Madison, Gallatin, and Upper Yellowstone River basins just experienced the ‘wettest’ April on record,” said NRCS water supply specialist Lucas Zukiewicz.

Most precipitation fell mid-month, when storms dropped wet snow on all but the lowest elevations. There, warmer temperatures and rain accelerated snowmelt.

The Columbia River basin, which includes all Montana rivers west of the Continental Divide, received 140 percent of the average precipitation for April. Even the Kootenai basin got 126 percent.

Yet snow in the Kootenai basin is only about 90 percent of average, similar to the St. Mary-Milk and lower Yellowstone river basins. This could push the Kootenai into another drought year, although spring rains may yet make a difference.

“A wet May and June could help to offset the deficits in snowpack and water-year precipitation we have in some areas, but given the uncertainty in the weather patterns this winter and spring, it’s a guessing game as to what will actually happen,” Zukiewicz said.

Mountain snowpack usually hits its peak in April, so this report is the best indicator of what’s to come in the way of summer streamflows. The only variable at this point is whether rainstorms might bump river levels up.

That’s part of what happened with the minor flooding in mid-April. Cold temperatures in February and March caused valley and low-elevation snow to stick around longer than normal. The snow kept April rains from soaking into the ground, so much of the runoff ended up in the rivers. That and a few warm days caused the Clark Fork to briefly swell above flood stage before receding.

The past few weeks of warmer weather have been enough to start melting mid-elevation snow. So Montanans should expect rivers to rise again, but it’s hard to predict how high they’ll rise or when that might occur.

Although rivers depend on the gradual melting of snow, rains and alternatively drought and heat also factor into river levels. For example, in 2017, the year of the Lolo and Rice Ridge fires, Montana experienced a “flash drought” in the middle of summer which caused streams to drop rapidly.

Zukiewicz doesn’t know how best to factor those in this year, even though May and June are often the wettest months, because climate change is making long-range weather predictions somewhat uncertain. That makes his streamflow predictions for May through July less certain.

With the information available at this point, streams in the Kootenai basin are predicted to have 80 percent of their average flows and most of those in the Flathead will run at 85 percent of average through July.

The Clark Fork River is predicted to have normal flow, except below Plains where the Flathead River joins in. The Bitterroot River is also predicted to be normal except on the West Fork, which will be 90 percent of average.

“This uncertainty in future weather, and just how much precipitation we will receive, is why our forecasts are issued as a range of outcomes and not just one hard number,” he said. “Even though we now have a good idea of the potential snowmelt component to this year’s runoff, it seems like we’re always playing the waiting game.”

Contact reporter Laura Lundquist at lundquist@missoulacurrent.com.