On May 11, 2018, the Clark Fork River above Missoula reached 33,250 cubic feet per second (cfs), the highest flow recorded since the river reached 48,000 cfs on June 1, 1908. This year’s peak flow was driven almost purely by the rapid snowmelt from the abundant and anomalous snowpack across the basin, unlike other big peak years (1964 and 1975) when there was a significant rain event in addition to already occurring snowmelt.
“This year will stand out in history as one of the biggest years on record for purely snowmelt-driven flows in rivers across the state,” said Lucas Zukiewicz, USDA Natural Resources Conservation Service (NRCS) water supply specialist for Montana. “The flows we experienced during May were a direct result of the well above normal to record-breaking snowpack in place before snowmelt began along with above average temperatures and abundant sunshine.”
While most rivers didn’t set new records for peak flows with regards to an instantaneous flow measurement (cfs), 52 stream gauges along rivers and streams set new records for May monthly flows, and 12 additional sites were the second highest on record. “Monthly flow refers to the total volume of water to move through the river during the month, and what we experienced during May was record-setting,” said Zukiewicz. Records at some stream gage locations go back 90 years. It wasn’t just in northwest Montana where new records were set. “Records for May flows span the entire state. Almost two-thirds of the gauges we forecast set new records, both east and west of the Divide,” said Zukiewicz.
The impacts of the rapid snowmelt may be felt later in the summer season. Ideally, the slow release of mountain snowpack provides long-duration flows in the rivers and streams across the state. This year, snowpack peaked well above normal in many basins, but once it peaked it came out at an accelerated rate. “Snowmelt rates this May were well above average throughout the month, and it took its toll on the snowpack,” said Zukiewicz.
Snowpack as of June 1 remains near to above normal at many high elevation snowpack monitoring locations, but most mid-elevations are below normal for this date, and low elevations have melted during the early half of May. “Based on the predominant weather patterns that we experienced this winter, cool and wet, this is not what we were expecting. It’s almost like we skipped spring altogether this year and went straight into summer. The snowpack is moving out quickly, and early, this year,” said Zukiewicz. Snowmelt-driven peak flows have likely occurred on many river basins across the state, but higher elevation driven river basins in south-central Montana could still see additional peaks.
As of this date, many reservoirs are full or are reaching capacity from the abundant runoff, which will help to sustain flows in many river basins later this summer. Summer streamflow forecasts issued by the USDA-NRCS Montana Snow Survey generally remain near to above average for the June 1 – September 30 period, but summer precipitation, especially in June, will play a critical role in determining the long-term water supply.
Monthly Water Supply Outlook Reports can be found at this website after the 5th business day of the month:https://www.nrcs.usda.gov/wps/portal/nrcs/mt/snow/waterproducts/basin/