Atmospheric River Brings Significant Flows to the Flathead Wild & Scenic Rivers

Written By Sheena Pate

December 8th-11th, 2025, an atmospheric river event delivered prolonged, moisture-rich storms across Northwest Montana, driving rapid runoff and notable rises on the Middle Fork and North Fork of the Flathead River. Both rivers peaked in the early morning hours of December 12th, 2025. Flathead Rivers Alliance tracked conditions throughout the event, monitoring USGS gage data, precipitation totals, and on-the-ground impacts to better understand how these powerful systems affect our Wild & Scenic rivers.

This post summarizes the hydrologic response, places the event in historical context, and highlights why continued monitoring matters for river health, public safety, and long-term stewardship.

Large woody debris floats towards Belton Bridge on the Middle Fork of the Flathead Wild & Scenic River, near West Glacier, MT.

What Is an Atmospheric River?

Atmospheric rivers are long, narrow corridors of concentrated water vapor that can transport immense amounts of moisture from the Pacific Ocean inland. When these systems stall over mountainous terrain like the Flathead Range, the resulting precipitation can quickly translate into elevated river flows, especially when rain falls on existing snowpack.

Event Snapshot: Key Hydrologic Statistics

(All provisional data from USGS gage stations unless otherwise noted. Updates may be made as additional information becomes available.)

Middle Fork Flathead River

  • Monitoring Site: USGS 12358500 M F Flathead River near West Glacier MT

  • Peak flow: 20,700 cfs recorded Dec. 12, 2025 01:15:00 AM MST *Discharge was 20,600 cfs at 01:10:00 AM MST and 01:30:00 AM MST

  • Pre-event flow: 1,140 cfs Dec. 8, 2025 2025 11:45:00 AM MST

  • Percent Increase: River discharge increased by approximately 1,716%, nearly an 18-fold (almost 20×) rise over 85.5 hours, climbing from 1,140 cfs on December 8, 2025 to 20,700 cfs by early December 12, 2025.

  • 2025 Spring peak flow: ~12,400 cfs May 12, 2025 15:00:00 PM MDT
    Data sourced: M F Flathead River near West Glacier MT - USGS Water Data for the Nation

  • Historical comparison:

Graphs: Middle Fork hydrographs with hydrologic event marked

North Fork Flathead River

North Fork Hydrographs with Hydrologic Event Highlighted

Precipitation & Snowpack Context

  • Storm total precipitation: ~4.6 inches over 72 hour precipitation total recorded, at Flattop Mountain SNOTEL site, as of Dec. 12, 2025.

    • Flattop Mountain SNOTEL is the primary station used for the Middle Fork headwaters because its watershed drains into McDonald Creek and directly into the Middle Fork Flathead River.

    • Pike Creek and Emery Creek provide additional context for spatial variability within the broader Flathead headwaters, including Middle Fork tributaries.

  • Rain-on-snow contribution: Significant

USDA Natural Resources Conservation Service (NRCS). Flattop Mountain SNOTEL Station Data. Available online: https://wcc.sc.egov.usda.gov/nwcc/site?sitenum=482&state=mt

Additional Data Resources:

How This Event Compares Historically

While the atmospheric river did not reach record-breaking flood levels, it stands out for:

  • The speed of the rise

  • Above-average flows for this time of year

  • A clear example of rain-on-snow dynamics, which can amplify runoff

Observed River & Community Impacts

During and immediately following the event, Flathead Rivers Alliance staff and partners observed:

  • Storm appeared to concentrate in the Middle Fork drainage

  • Elevated turbidity and sediment transport

  • Bankfull or near-bankfull conditions in select reaches

  • Limited overbank flooding in low-lying areas

  • Increased debris movement, including large woody material

Winter flooding in Montana rivers brings both challenges and benefits. While high flows can wash out fish eggs and reshape river habitats, they also play a critical role in maintaining healthy rivers over the long term. Floodwaters help clear sediment, form new gravel bars for spawning, improve habitat connectivity by redistributing logjams and nutrients which ultimately supports thriving trout populations once the waters recede and clear.

Why Monitoring Matters

Tracking events like this atmospheric river helps us:

  • Provide timely, science-based information to the community

  • Understand how climate-driven shifts may influence future flow regimes

  • Advocate for resilient river corridors and floodplain connectivity

  • Inform recreation safety and infrastructure planning

As atmospheric rivers become more variable and potentially more intense long-term datasets from the Middle and North Forks are essential for protecting the Wild & Scenic character of the Flathead River system.

Stay Engaged

Flathead Rivers Alliance will continue to monitor river conditions and share updates during high-water events and throughout the year. You can stay connected and support this work by taking a few simple actions:

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Together, we can ensure the Flathead’s Wild & Scenic rivers remain healthy, resilient, and free-flowing for generations to come.

CONGRATULATIONS to @mountainsandmontana on IG for guessing 20,600 cfs peak streamflow for Middle Fork of the Flathead Wild & Scenic River @ West Glacier USGS gage station. They were closest to the recorded peak streamflow at that site.

Sheena Pate
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