Ice Age Floods National Geologic Trail
Ice Age Floods National Geologic Trail
Thompson Falls State Recreation Area in northwestern Montana occupies a site that lay beneath roughly 2,000 feet of water when Glacial Lake Missoula was at its maximum extent -- deeper than the ocean off most continental shelves. The lake formed when the Purcell Trench Lobe of...
Thompson Falls State Recreation Area in northwestern Montana occupies a site that lay beneath roughly 2,000 feet of water when Glacial Lake Missoula was at its maximum extent -- deeper than the ocean off most continental shelves. The lake formed when the Purcell Trench Lobe of the Cordilleran Ice Sheet blocked the Clark Fork River near Lake Pend Oreille, Idaho, impounding water that filled the Clark Fork valley for over 200 miles upstream. At Thompson Falls, the lake stretched from mountainside to mountainside, its surface so high that only the tallest peaks protruded above the waterline. The modern Thompson Falls on the Clark Fork River cascades over a bedrock ledge that was likely scoured and shaped by the repeated filling and draining of the lake. Sediment deposits in the surrounding valley record the rhythmic cycles of lake formation and catastrophic drainage that occurred 40 or more times over roughly 2,500 years. Standing at the recreation area, look up at the mountainsides and try to imagine the water level at their peaks: that is how deep this valley was filled during the ice age, a fact that still stuns geologists today.
Thompson Falls State Park is on the Clark Fork River in northwestern Montana, about a mile north of the town of Thompson Falls on Highway 200. Open year-round with campground, day-use areas, and river access; standard Montana State Parks fees apply (free for Montana residents with vehicle registration; $8 per non-resident vehicle).
Thompson Falls sits in the upper Clark Fork corridor — directly within the basin of Glacial Lake Missoula. The town is at approximately 2,400 feet elevation; the lake's high shoreline reached roughly 4,200 feet, putting Thompson Falls under as much as 1,800 feet of water at lake maximum. Each time the ice dam at the mouth of the Clark Fork failed (estimated at 40+ times over the period roughly 18.2–14.0 ka, anchored by Balbas et al. 2017), water from the lake passed through this corridor on its way to the breach. Thompson Falls is therefore a lake-bottom site rather than a flood-erosion site — the geologic story here is about ponding and rapid drainage, not channel scour. Visible features include slackwater silt benches and high lake-strand evidence on nearby ridges.
No site-specific publications located. Glacial Lake Missoula chronology and shoreline-mapping work has continued through Montana FWP and academic partners; the USGS review (O'Connor and Baker 2020) is the standard recent synthesis.
No dedicated chapter at the park. The Glacial Lake Missoula Chapter (Missoula) covers this corridor and uses the Clark Fork drive as a programming framework.
Spring through fall for fishing, paddling, and camping; the park is reservable but uncrowded relative to Glacier National Park traffic. Drive Highway 200 east toward Paradise/Plains for additional flood features (kolks, gulch fills, and the Paradise Center).
Every site along the trail will receive the full Terrain360 capture treatment: ground-level 360° panoramas, drone aerial imagery, and photogrammetry-based 3D models that visitors can spin in their browser. This page reserves the slots; the imagery flows in as field capture completes.
Ground-level 360° panorama, every step along the feature, captured by Terrain360 field crews.
Drone flyovers reveal the geometry of catastrophe — ripple marks, gravel bars, and scour patterns invisible from the ground.
Photogrammetry and Gaussian-splat models let visitors rotate, measure, and inspect features in detail-page WebGL viewers.