Ice Age Floods National Geologic Trail
Ice Age Floods National Geologic Trail
Dog or Rainbow Lake is clearly dammed by moraine at the West. That this moraine came from the same ice as that on the Boyer/Lynch divide Bretz Field Notes
Dog or Rainbow Lake is clearly dammed by moraine at the West. That this moraine came from the same ice as that on the Boyer/Lynch divide Bretz Field Notes— Bretz 1929
This is one of more than 1,800 field sites cataloged in the early scabland surveys — the bedrock of the Ice Age Floods scientific record. The Bretz-era researchers walked the ground first; modern cosmogenic dating, LiDAR, and remote sensing have since extended and refined what they mapped.
Western Montana is where the Ice Age Floods began. The region centers on Missoula and stretches northwest down the Clark Fork drainage to the Idaho line, north into the Flathead and Mission valleys, and south up the Bitterroot. Specific features in scope here include the city of Missoula and its bracketing peaks (Mount Sentinel, Mount Jumbo); the Clark Fork Valley downstream through Thompson Falls to Cabinet Gorge near the Idaho border; the Camas Prairie and its Glacial Lake Missoula National Natural Landmark; the Bitterroot Valley and the granite erratics scattered across its floor; the Mission Mountains and Mission Valley; Polson and the Polson Moraine at the south end of Flathead Lake; and the Bison Range, whose lower slopes were inundated and whose upper ridges stood as islands. All of these landscapes sit within, or immediately upstream of, the basin that filled and emptied to produce every Missoula Flood.
Roughly 15,000 to 21,000 years ago, the Purcell Trench Lobe of the Cordilleran Ice Sheet pushed south out of British Columbia along what is now the Lake Pend Oreille trench in the Idaho Panhandle. Where it crossed the Clark Fork River near the present Idaho-Montana line, it stopped the river cold. The ice mass at the dam was about 2,000 feet thick and extended at least 10 miles upstream — by some estimates as much as 30 miles. Water backed up behind it.
The lake that formed, Glacial Lake Missoula, filled the Clark Fork, lower Bitterroot, and Flathead valleys. At maximum stand it held more than 500 cubic miles of water — more than Lake Erie and Lake Ontario combined — with a surface area of about 3,000 square miles and a depth of roughly 2,000 feet at the dam. The lakebed reached into Missoula, the Bitterroot Valley, the Mission Valley, and across the lower slopes of the Bison Range. The valley floor of present-day Missoula sat under nearly 2,000 feet of water; the tops of Mount Sentinel and Mount Jumbo stood as islands above the surface.
The dam did not hold. As water rose to roughly 600 meters depth at the ice, the ice became buoyant and water forced its way along the glacier bed in a catastrophic subglacial discharge — a jokulhlaup, the same mechanism that drains modern Icelandic ice-dammed lakes. Either flotation, tunneling, or thermal erosion (or all three in sequence) blew out the dam. Drainage was fast: peak discharge at the lake's outlet has been reconstructed at roughly 20 million cubic meters per second, and the lake may have lost most of its 380 cubic miles of usable water in days. The dam then re-formed as the ice readvanced, the lake refilled, and the cycle repeated.
That cycle ran roughly 40 times over about 2,000 to 4,000 years near the end of the last glaciation. Early in the lake's history the fill-and-flush interval may have lasted a century; for the last several floods it appears to have shortened to one or two years. Varves, paleomagnetic secular variation, and rhythmite counts on the downstream side support that frequency.
The strand lines on Mount Sentinel and Mount Jumbo are the most legible record of this in plain sight. Each time the lake stood at a stable level long enough for waves to cut a shoreline notch, it left a horizontal line on the slope. More than 30 distinct strand lines are visible — in low sun, in snow contrast, or where vegetation changes — and they mark the multiple lake stands as the dam broke and reformed. They are the bathtub rings of a lake that no longer exists, etched into the two peaks that frame the city of Missoula.
Downstream, where lake water poured through Eddy Narrows on the Flathead River and out onto the Camas Prairie, currents reshaped the valley floor into one of the most distinctive flood-bedform fields on Earth. The Camas Prairie giant ripples are ridges 15 to 50 feet high, 100 to 250 feet wide, and up to half a mile long. They are giant flood ripples — sedimentary structures identical in form to ordinary stream ripples, but two to three orders of magnitude larger, formed by water moving at roughly 60 miles per hour through 1,000 feet of depth. The site has been a National Natural Landmark since 1966.
In the Bitterroot Valley, the evidence is different but equally direct. Tributary alpine glaciers descended from the Bitterroot, Mission, and Rattlesnake ranges and reached the shoreline of Lake Missoula. When the lake was deep, the lower ends of those glaciers floated as icebergs, carrying entrained boulders of Bitterroot quartz monzonite (and other rock) out into the lake. As the icebergs drifted and melted, they dropped those boulders onto the lake floor. That lake floor is now the valley floor. The Bitterroot Valley erratics — quartz monzonite blocks sitting 5 to 20 miles from any possible source — are the physical record of ice-rafted deposition. Western Montana is the source of every Missoula Flood. Everything downstream — the Channeled Scablands, Wallula Gap, the Columbia Gorge, Willamette erratics — begins with the dam breaking near Cabinet Gorge and the lake rushing out across Camas Prairie.
J Harlen Bretz worked out the flood story from downstream evidence in the Channeled Scablands beginning in 1923, but he could not name a water source large enough to do the work. That gap is what made his hypothesis unacceptable to the geological establishment for almost two decades. Joseph T. Pardee, a USGS geologist who had been mapping in western Montana since the early 1900s, was the one who closed it.
Pardee published a short paper in 1910 noting the lake deposits around Missoula and identifying their glacial origin. He had recognized that Lake Missoula had existed at scale, and in correspondence with Bretz in the 1920s he appears to have considered — privately — that the lake's drainage could be the source Bretz was missing. His 1942 paper in the Geological Society of America Bulletin, "Unusual Currents in Glacial Lake Missoula, Montana," made it public. Pardee described the Camas Prairie ridges as giant current ripples produced by catastrophic flow, reconstructed velocities of about 45 miles per hour and discharges of roughly 386 million cubic feet per second through the outlet, and explicitly tied the lake's outburst drainage to the scabland features Bretz had documented. After Pardee's 1942 paper, general acceptance of the megaflood hypothesis followed, though it took another 20 years to become consensus. Bretz himself credited Pardee in later correspondence.
Work since the 1980s has refined three things: how many floods, how often, and when.
Richard Waitt's rhythmite work in southern Washington identified about 40 superposed flood beds, giving a minimum count of 40 last-glacial fillings and emptyings. Varve thicknesses within rhythmites suggest fill-and-flush intervals of around 50 years on average, declining from possibly a century early in the lake's history to one or two years for the latest floods. Optical luminescence dating at Garden Gulch (Smith and colleagues, 2018) constrains the timing of the deep last-glacial lake stands, showing the lake reached more than 65 percent of maximum capacity by about 20.9 ka and drained from that position multiple times.
Balbas et al. (2017, Geology) provided the first direct cosmogenic ages on flood-scoured bedrock. Using 10Be exposure dating on flood and glacial features, they showed one of the largest floods occurred at 18.2 plus or minus 1.5 ka, traveling down the northwestern Columbia River valley before the Okanogan Lobe blocked that route and diverted later floods east through the Channeled Scabland. The youngest dated floods, at 14.0 to 14.4 ka, came down the same northwestern route from Glacial Lake Columbia, indicating that lake persisted for centuries after the last Missoula flood. The 18.2 ka anchor is now the standard chronological reference for the main flood sequence.
Paleohydraulic reconstructions have also tightened. Reconstructions of the largest drainages (Denlinger and colleagues, building on earlier hydraulic work) put peak discharge at the Lake Missoula outlet near 20 million cubic meters per second, with about 5 to 15 million cubic meters per second through Wallula Gap and the Columbia Gorge. Ice-dam failure mechanics are best described as a self-dumping ice-dammed lake — the jokulhlaup model — driven by hydrostatic flotation once water depth at the dam reached about 600 meters.
Missoula is the practical base. The strand lines on Mount Sentinel (the "M" hill) and Mount Jumbo (the "L" hill) are visible from anywhere in town, and public trails switchback through them on both peaks. The Montana Natural History Center, at 120 Hickory Street near McCormick Park, runs the standing exhibit "Explore Glacial Lake Missoula" and partners with the Ice Age Floods Institute's Glacial Lake Missoula Chapter on programming, field trips, and the chapter's network of engraved high-water-mark stones placed at the 4,200-foot lake-stand elevation around the city. The chapter meets at the center monthly as needed.
The Glacial Lake Missoula National Natural Landmark sits about 68 miles northwest of Missoula on Montana Highway 382, just over Markle Pass and 12 miles north of Perma, on the Camas Prairie east of the highway. Driving Highway 382 south over Markle Pass into Camas Prairie is the textbook approach — the ridges read as wrinkles in the grassland from the pass, and become unmistakable on the prairie floor. Guided tours of the ripples operate seasonally out of Hot Springs. Downstream, the Cabinet Gorge area near the Idaho line, Eddy Narrows on the Flathead, the Polson Moraine at the south end of Flathead Lake, and the Bison Range all sit within the former lakebed and are reachable on a one- or two-day driving loop from Missoula.
Active: 1914-1979 (primary IAF fieldwork 1922-1932; Penrose Medal 1979) Affiliation: University of Chicago (Instructor 1914, Professor 1926, retired 1947, emeritus through 1979) Key paper: Bretz, J H. (1923). "The Channeled Scabland of the Columbia Plateau." Journal of Geology 31(8): 617-649.
Bretz spent seven summer field seasons beginning in 1922 walking the basalt coulees, dry cataracts, and giant gravel bars of eastern Washington and concluded that the landscape required a catastrophic flood at a scale no existing process could produce. He coined the term "Channeled Scabland" in his 1923 paper and over the next decade defended the hypothesis against a uniformitarian geology establishment that had no plausible source for the water and viewed catastrophism as scientifically illegitimate. Bretz refused to name a source until Pardee's 1942 paper supplied one: Glacial Lake Missoula. He continued publishing on the scablands as an emeritus professor through the 1950s and 1960s, including his synthesis "Washington's Channeled Scabland" (1959). The Geological Society of America awarded him the Penrose Medal in 1979 at age 96; he reportedly remarked to his son, "All my enemies are dead, so I have no one to gloat over."
Source: J Harlen Bretz - Wikipedia; HistoryLink.org; University of Chicago Magazine
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.
Today we know the Purcell Trench ice lobe created an ice dam roughly 2,000 feet tall at this latitude. The dam formed and broke repeatedly over 2,000 years, producing as many as 80 separate catastrophic floods.