Ice Age Floods National Geologic Trail
Ice Age Floods National Geologic Trail
Rainbow Lake is a 1.5-mile-long lake along Montana Highway 200 that fills a channel scoured by the catastrophic drainage of Glacial Lake Missoula. The lake occupies a kolk-scoured depression -- a hole drilled into bedrock by the violent spinning vortices that formed when 500...
Rainbow Lake is a 1.5-mile-long lake along Montana Highway 200 that fills a channel scoured by the catastrophic drainage of Glacial Lake Missoula. The lake occupies a kolk-scoured depression -- a hole drilled into bedrock by the violent spinning vortices that formed when 500 cubic miles of water drained through this valley in a matter of days. The Clark Fork River valley at this point was a constricted corridor that focused the draining lake's energy, and the floodwaters reached speeds estimated at 60 to 80 miles per hour through these narrow sections. The depth of the water here during drainage may have exceeded 1,000 feet, and the turbulent currents generated kolks that excavated depressions now filled by lakes like Rainbow. The surrounding valley walls show scour marks and stripped vegetation lines consistent with catastrophic drainage. The lake's elongated shape, aligned with the valley axis, records the direction of the flood flow. Driving along Highway 200 between Plains and Thompson Falls, you pass through what was essentially the barrel of a water cannon -- and Rainbow Lake is one of the bullet holes.
Rainbow Lake (also called Dog Lake) is in western Montana and is documented as Tour Stop D on the Bjornstad/Kiver hugefloods.com tour route for the Ice Age Floods National Geologic Trail. Public-access details are not well documented on the IAFI or NPS sites; the surrounding terrain is largely undeveloped. This is not a developed visitor site — it is a roadside or short-walk-in stop for tour groups.
Rainbow Lake sits within Glacial Lake Missoula's eastern footprint. The site is notable for angular boulders ripped from local bedrock and deposited near the lake — angular because they were not tumbled in flood transport but plucked and dropped at short range, a signature of high-energy erosion close to the source rather than far-traveled flood deposition. The site is one of the few places where the lake's eastern hydraulic behavior, rather than the downstream Columbia Plateau outburst story, is on display. As Lake Missoula's water level rose, a second outlet briefly developed at roughly 3,588 feet elevation through the Rainbow Lake area before the main Clark Fork ice dam failed.
Coverage in Bjornstad and Kiver's On the Trail of the Ice Age Floods and on Bjornstad's hugefloods.com site is the principal interpretive source. No formal peer-reviewed updates found since 2017.
Falls within the IAFI Glacial Lake Missoula Chapter's coverage area but is not a heavily programmed site.
Best treated as part of a guided IAFI field trip rather than an independent visit; access roads and any trails are not well signed. Bring a copy of Bjornstad's tour route if attempting independently.
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.