Ice Age Floods National Geologic Trail
Ice Age Floods National Geologic Trail
Every drop of water in Glacial Lake Missoula -- 500 cubic miles of it -- had to funnel through this 10-mile-long canyon between Plains and Thompson Falls, and the evidence of that passage is carved into the bedrock. Eddy Narrows is where the northern and eastern arms of the lake...
Every drop of water in Glacial Lake Missoula -- 500 cubic miles of it -- had to funnel through this 10-mile-long canyon between Plains and Thompson Falls, and the evidence of that passage is carved into the bedrock. Eddy Narrows is where the northern and eastern arms of the lake converged and accelerated through a constricted stretch of the Clark Fork River valley, reaching speeds that geologist J.T. Pardee calculated at nearly 80 miles per hour. Pardee used the dimensions of this flume-shaped canyon to estimate that Glacial Lake Missoula could have emptied in as little as three days -- a calculation that stunned the geological community when published in 1942. The steep valley walls are stripped bare of talus and soil up to the estimated 1,000-foot lake level, and polished grooves in bedrock benches 400 feet above the Clark Fork run parallel to the river, scoured by the rushing water. There are pullouts along MT-200 at either end of the narrows, with Glacial Lake Missoula interpretive signage near milepost 59. Stand here and imagine the entire contents of two Great Lakes being forced through this canyon in three days.
Eddy Narrows is a 10-mile canyon reach of the Clark Fork River between Plains and Thompson Falls, Montana, paralleled by MT Highway 200. There is no formal visitor facility — viewing is from highway pull-offs and the BNSF right-of-way is off-limits. Year-round road access; winter weather can be hazardous.
Eddy Narrows acted as the "flume" through which Glacial Lake Missoula drained during each ice-dam failure. Pardee's 1942 reconstruction used the canyon's restricted cross-section to back-calculate flow velocities approaching 80 mph, with peak discharge near 10 cubic miles per hour and roughly 380 cubic miles of water passing through during a single drainage event — emptying the lake in as little as three days. Polished bedrock grooves on a bench 340–400 ft above the present river, oriented parallel to flow, and the absence of talus or soil up to roughly 1,000 ft above the river mark the floodwater scour zone.
The Balbas et al. 2017 cosmogenic chronology constrains the largest flood events to 18.2 ± 1.5 ka and 15.6–14.7 ka. Larry Smith's 2021 Glacial Lake Missoula field guide (Montana Tech) revisits Pardee's discharge calculations and the Eddy Narrows evidence without major revision. No major reinterpretation found since 2017.
The Glacial Lake Missoula chapter (Montana) covers Eddy Narrows and discusses it in field-trip materials. No on-site panel.
Year-round drive-by viewing. Best when low spring/summer sun rakes the scour-zone bench across the river. Pair with Camas Prairie Ripples and the Glacial Lake Missoula NNL for a regional driving loop.
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.