Low winter moisture puts Clark Fork, Blackfoot rivers at half flow

Laura Lundquist

(Missoula Current) As June settles in and Montana experiences its first few days of above-80 degrees, the chance of the mountains receiving any more snow disappears, as does the snow. For western Montana, that means rivers and streams will drop low.

On Friday, the US National Resources Conservation Service released its final monthly water supply outlook report, which confirmed that much of western Montana is still looking dry, in spite of the moisture and cool weather that accompanied May.

“Last month brought a mix of snow, rain, and sunshine across Montana. While that is not uncommon weather for this time of the year, some locations did not receive the moisture they needed,” said Eric Larson, USDA Natural Resources Conservation Service (NRCS) Water Supply Specialist.

Most river basins received greater-than-average precipitation during May, although the far western basins of the Kootenai and Lower Clark Fork below Missoula received only 80-90% of average precipitation. But that wasn’t enough to overcome the lack of moisture that western Montana experienced during the course of the winter. The total precipitation for all basins west of the Continental Divide and east along the Front was 80-90% of normal. The large basins in the central and eastern regions of the state received precipitation amounts close to the 30-year average.

While some of that precipitation filters down as snow, not enough of that happened this year. At the end of May, the Bitterroot and Upper Clark Fork river basins had about 65% of the snowpack averaged over the past 30 years. The Lower Clark Fork and Flathead basins were slightly better at 72% and 83% of average. Most of the basins east of the Continental Divide ended up with average or better snowpack except for the Upper Missouri and Sun-Marias-Teton basins. Those two ended up with alarmingly low snowpack between 3 and 16% of the 30-year average.

At many SNOTEL sites, particularly those at lower elevations, the peak snowpack didn’t amount to much. In western Montana, 11 sites from the Rocky Mountain Front west of Browning to the Tobacco Root Mountains south of Whitehall had record-low peak snowpack and another four had near-record-lows. But a few high-elevation sites including a few in the northern Swan Range benefitted from the late snows in April and May and peaked close to their average snowpack.

“Keep in mind, snowpack percentages were well below normal most of this season, particularly around May 1 when it peaked for the year. Current snowpack percentages are closer to normal than what might be expected because of delayed melt at the highest elevations,” Larson said.

But low precipitation total and low snowpack mean that streams don’t have much to draw on especially as the summer wanes, although they’re doing better than what was anticipated a month ago. The Jefferson, Sun, Teton, Marias, Upper Clark Fork, and Blackfoot are forecasted to have only about 50-60% normal streamflow through July. The Clark Fork River passing through Missoula will likely suffer higher algae growth as the water dwindles and warms.

The Flathead, Yellowstone, Gallatin and Madison basins still have a substantial amount of snow remaining at the highest elevations so they are now forecasted to have about 80-90% of normal streamflow through July. Streamflow forecasts for the Smith, Judith, and Musselshell rivers improved but will probably be about 70% of normal through July.

“Below-normal snowpack peak levels this season will likely have an impact on streamflow later this summer. From a water supply perspective, above-normal precipitation during the summer is almost always welcomed, and that is certainly true this year in Montana,” Larson said.

More precipitation would be helpful, but the three-month outlook from the National Oceanic and Atmospheric Administration doesn’t look good. From July through September, the precipitation has a 50% chance of being lower than normal across the state while the temperature is predicted to be above normal, especially on the western side of the state.

The western side of the state is the region still suffering the most from drought, according to the U.S. Drought Monitor. Most of the western side is in moderate drought with a swath of the Bob Marshall Wilderness Complex dipping into severe drought.

Contact reporter Laura Lundquist at lundquist@missoulacurrent.com.