Ice Age Floods National Geologic Trail
Ice Age Floods National Geologic Trail
Friend stands just west of the limits of CRB outcrops. Material westward for 6 miles is volc tuff Bretz Field Notes
Friend stands just west of the limits of CRB outcrops. Material westward for 6 miles is volc tuff Bretz Field Notes— Bretz 1920
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.
This region covers the great hydraulic bottleneck of the Ice Age Floods: the reach of the Columbia River from the lower Snake/Columbia confluence near the Tri-Cities (Pasco, Kennewick, Richland), south through Wallula Gap, and then west through the entire Columbia River Gorge to the head of the Portland Basin. It includes the Pasco Basin and its tributary embayments (Yakima Valley, Walla Walla Valley, Touchet and Tucannon valleys, Quincy Basin to the north), the Hanford Reach and White Bluffs along the unimpounded Columbia, Hat Rock and the lower Umatilla Basin downstream of Wallula, and the Gorge from The Dalles past Rowena Crest, Mosier, Hood River, Cascade Locks and Bonneville, exiting at Crown Point above the Portland forearc basin. Almost every flood that reached the Pacific passed through this corridor, no matter which route it took across the Channeled Scabland upstream. This is the single reach where every Missoula flood path converges.
Wallula Gap is the choke point of the whole system. The gap is a roughly 1.2 km (2 km) wide notch where the ancestral Columbia cuts through the basalt anticlines of the Horse Heaven Hills. When floodwater arrived in the Pasco Basin from the Cheney-Palouse, Telford-Crab Creek, and Grand Coulee tracts, peak discharge dwarfed what the gap could pass. O'Connor and Baker's classic 1992 paleohydraulic reconstruction, refined in the 2020 Earth-Science Reviews synthesis (O'Connor, Baker, Waitt, Smith, Cannon, George, and Denlinger), puts peak discharge at Wallula in the 5-15 million m³/s range, against a Pasco-Basin-inlet supply that may have approached 20 million m³/s. The gap simply could not pass water as fast as it arrived. The result was a temporary lake, Lake Lewis, that pooled behind the constriction.
Lake Lewis was not one body of water but a flood-cycle phenomenon, refilled with every release of Glacial Lake Missoula. At peak stage it stood near 1,200 feet (about 365-370 m) above modern sea level. It drowned the Pasco Basin, backflooded west up the Yakima River past the present city of Yakima (about 250 feet deep there), east up the Walla Walla, Touchet, and Tucannon valleys, and north into the Quincy Basin. Tri-Cities sat under roughly 900 feet of water. The lake lasted only days to a few weeks per flood while water bled south through the gap, then drained and refilled the next time the ice dam on the Clark Fork failed. Sediment settled out of the slackwater arms during each filling, building the rhythmically bedded Touchet Beds.
The Touchet Beds are the highest-resolution physical record of repeated flooding anywhere in the system. In Burlingame Canyon west of Walla Walla, Richard B. Waitt counted at least 41 distinct rhythmites in a single exposure and argued in his 1980 Journal of Geology paper ("About forty last-glacial Lake Missoula jokulhlaups through southern Washington") that each represented a separate jokulhlaup. Up-valley thinning, up-valley paleocurrents, up-valley transport of Cordilleran-derived ice-rafted erratics, and graded bedding (coarse base, fine top) all show these are backflood deposits, not normal lacustrine sediment. The accompanying clastic dikes, sheeted wedge-shaped intrusions cutting down through more than a dozen geologic units, record loading and dewatering during repeated rapid inundation.
The 18.2 ka anchor comes from Wallula itself. Balbas, Barth, Bierman, Caffee, Rood, and Zimmerman (Geology, 2017) collected 10Be samples from an ice-rafted granitic erratic at 341 m elevation at Wallula Gap, the only outlet for Lake Lewis, and reported a cosmogenic exposure age of 18.2 +/- 1.6 ka. Companion samples on the high Columbia valley walls near Wenatchee yielded statistically indistinguishable ages of 18.2 +/- 1.5 ka. Because that erratic sits near the highest preserved flood stage at the outlet, the date directly fixes the timing of a flood that filled Lake Lewis to near its peak elevation. It is the most defensible single age for peak flood depth in the entire flood system.
Downstream of Wallula the water re-accelerated through the Umatilla Basin past Hat Rock, a 70-foot resistant Columbia River Basalt plug stripped clean by the floods and the first feature Lewis and Clark noted by name when descending the Columbia in 1805. From there flow entered the Gorge, where it became hydraulically supercritical at constrictions near Crown Point and Mitchell Point. The Gorge's basalt walls were undercut and steepened, hanging tributary valleys were truncated, and waterfalls like Multnomah, Latourell, Bridal Veil, Wahkeena, Horsetail, and Oneonta were either created or substantially enhanced where small streams now plunge over the over-steepened main-stem walls. A second transient lake, Lake Condon, formed in The Dalles basin behind the Rowena chokepoint. At Crown Point, peak flood stage reached roughly 700 feet, near the elevation of Vista House itself. West of the Gorge, the floods backed up behind a constriction at Kalama Gap into the Portland Basin and Willamette Valley (Lake Allison), depositing the Willamette Silt and dropping ice-rafted erratics as far south as Eugene.
J Harlen Bretz began his Channeled Scabland fieldwork in the summer of 1922 and returned every summer through 1931, typically with University of Chicago graduate students. His traverses included the Pasco Basin, Wallula Gap, the Walla Walla embayment, and the Columbia Gorge downstream to Crown Point and Bonneville. The Gorge was the half of the system that gave Bretz his deepest trouble with reviewers: critics could accept large floods in the Scabland but argued the Gorge had been cut by ordinary Columbia River flow over millions of years. Bretz's response was to show, feature by feature, that the Gorge walls were undercut to a height that ordinary discharge could not produce, that hanging valleys with overfit basalt cataracts (Multnomah, Horsetail, Latourell) recorded sudden over-deepening of the main stem, and that giant gravel bars from Crown Point west into the Portland Basin matched flood, not river, hydraulics. Bretz also walked the Walla Walla Valley exposures (later formalized as the Touchet Formation) but interpreted them initially as a single flood deposit. The "Spokane flood debates" of the late 1920s and 1930s, well summarized in Baker's 2008 review, pivoted around exactly this region, because the Pasco Basin and Gorge were where the evidence was hardest to argue away.
Three threads of work since 2010 have refined the picture without overturning it. First, the chronology has tightened. Balbas et al. 2017 anchored peak flood stage at Wallula to 18.2 +/- 1.6 ka via 10Be on the high erratic, and the Coyote Canyon Mammoth Site OSL chronology (Last and Rittenour, Quaternary, 2021) gave seven OSL ages on flood rhythmites entombing Columbian mammoth remains, ranging from 20.9 +/- 2.6 ka to 16.3 +/- 2.8 ka, with an internal radiocarbon constraint near 17.4 +/- 0.2 ka cal BP. Both datasets converge on the established picture: multiple floods between roughly 20 and 14 ka, with the earlier floods reaching higher stages. Second, hydraulic modeling has gone two-dimensional. Denlinger, George, and collaborators' models reported in the 2020 review reproduce flood stages reasonably well along most of the route but systematically underestimate peak stage at Wallula by as much as 130 feet, suggesting either that backwater coupling with The Dalles and Portland Basin is stronger than first models assumed, or that supply rates from upstream were higher than nominal Lake Missoula maximums. Third, the depositional record continues to be re-read. Work on the Touchet Beds clastic dikes (Cooley and others) and the rhythmite sequences exposed in the White Bluffs along the Hanford Reach has documented that the upper White Bluffs preserve roughly a dozen sandy rhythmites within their otherwise Plio-Pleistocene Ringold Formation channel-fill, capped by wind-blown sand recording inter-flood dryness.
Wallula Gap is a designated National Natural Landmark. US-730 runs through the gap along the east side of the Columbia, with a Twin Sisters basalt-spire pull-off providing the best view back upstream into the Pasco Basin and the Lake Lewis high-stand level on the valley walls. The IAFI Lake Lewis Chapter, organized in 2003, runs field trips into the Touchet Beds (Burlingame Canyon is the type exposure; access is via the Walla Walla Valley) and to Wallula. Hat Rock State Park near Umatilla, Oregon, gives the cleanest single view of a basalt residual stripped by the floods. The Hanford Reach National Monument preserves the only unimpounded stretch of the Columbia below Grand Coulee Dam and offers White Bluffs Overlook, where the Lake Lewis rhythmites are visible in the upper bluff face. The REACH Museum in Richland is the regional interpretive hub. In the Gorge, the IAFI Columbia River Gorge Chapter runs semi-annual bus and walking field trips of a roughly 40-mile section between Cascade Locks and Columbia Hills State Park; Crown Point/Vista House and Rowena Crest are the two best free overlooks for reading flood stage on the landscape.
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.
Modern flood modeling shows that water levels in the Gorge reached 800-1,000 feet above the current river, moving at estimated speeds of 60-80 mph. The high-water marks Bretz observed are now precisely mapped by LiDAR.