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...
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.
Three vantages no single photograph can hold, the same treatment that made Dry Falls legible.
Ground-level panoramas along the feature, so the scale of the flood landscape is something you stand inside.
An aerial reveals the geometry of catastrophe: scour, channels, and bars that are invisible at eye level.
A model of a key outcrop you can rotate and measure in the browser, the rock itself, on the page.
Capture window mid-June through mid-July 2026 · slots fill on this page as the campaign delivers.
Flood-extent overlay shows the maximum reach of the Missoula Floods.
The site in its place along the flood path, with the maximum flood extent drawn over the modern map.
View on the interactive map Cinematic timeline · 3D flood · every captured site