Ice Age Floods National Geologic Trail
Ice Age Floods National Geologic Trail
Andesite of Troutdale Point a typical fine grained rock of typical gray color. Columar in sharply terminated domes like arches of Gothic windows. Bretz Field Notes
Andesite of Troutdale Point a typical fine grained rock of typical gray color. Columar in sharply terminated domes like arches of Gothic windows. Bretz Field Notes— Bretz 1916
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 reach where every Missoula flood that survived the Columbia River Gorge spread out and lost its confinement. It begins at the mouth of the Gorge near Crown Point and the Portland Women's Forum overlook at Chanticleer Point, includes the Portland Basin proper (the forearc embayment occupied today by Portland, Vancouver, Gresham, Troutdale, Beaverton, and Hillsboro), the Tualatin Valley as a major slackwater embayment to the west and south, the narrow Lake Oswego-Tualatin connection that linked the two basins, the Tonquin Scablands south of Tualatin between Sherwood and Wilsonville, the lower Willamette confluence with the Columbia near present-day Kelley Point, and the Willamette mainstem south through Oregon City to the head of the Willamette Valley. High-water reference points around the rim include Rocky Butte and the Boring volcanic chain in east Portland, Mount Sylvania in the southwest Portland hills, and Mount Pisgah south of Eugene at the far upstream limit of backflooding. This is the reach where the floods stopped being a hydraulic event and became a depositional event, leaving the Willamette Silt that built the modern soil column.
The Portland Basin is where every flood path converged and then immediately had to slow down. Water exited the Gorge through a constriction near Crown Point and Mitchell Point that O'Connor and Baker's paleohydraulic reconstructions identify as a hydraulic control, where flow went critical and stage was pinned by channel geometry rather than by upstream supply. At peak stage the flood profile at Crown Point reached roughly 700 feet above modern sea level, near the elevation of Vista House. Once water passed Crown Point and the basalt walls fell away, the basin widened by roughly an order of magnitude. The flood lost confinement, decelerated, and dropped its coarsest load as the great Portland gravel delta, a body of bouldery sediment now exposed in cut banks and quarries from Troutdale west into east Portland.
Downstream of Portland, another constriction at Kalama Gap in southwest Washington acted as the second governing control on the whole lower system. Where the Columbia is forced through a narrow basalt-walled reach near present-day Kalama, the channel could not pass peak flood discharge any faster than upstream supply was arriving. Water backed up. The result was Lake Allison, the temporary lake that filled the Portland Basin and the entire Willamette Valley behind the Kalama constriction during every flood cycle. At its peak, Lake Allison stood at roughly 400 feet (about 120 m) above modern sea level. Portland, at a present elevation around 20 feet, would have been under 380 feet of water. Rocky Butte, an east Portland Boring-age cinder cone with a summit at 613 feet, was overtopped by something like 80 to 180 feet of water at peak stage, and its east face was extensively eroded by the floods. Mount Sylvania, in the southwest Portland hills, stood as a small island or partly submerged shoal in the lake and received ice-rafted erratics on its lower flanks. South of Portland, Lake Allison extended 200 km up the Willamette Valley to Eugene, where Mount Pisgah marks the upstream limit of backflooding on the rim of the valley. Ira S. Allison, for whom the lake is named, walked that rim in the 1930s and tracked a continuous ring of erratics on the lower hillsides at near-constant elevation, demonstrating a former lake surface.
The Tualatin Valley west of Portland acted as a side basin off the main Lake Allison body. The hydraulics were unusual. The Tualatin sits behind the low Tualatin Mountains, and the only easy connection to the Willamette is through the Lake Oswego-Tualatin gap, a narrow buried channel that the modern Tualatin River and Oswego Canal still occupy. During every flood, water from the Lake Allison high stand in the Willamette pushed west through the Lake Oswego connection, filled the Tualatin Valley as a quiet backwater, and then drained back out through the same gap as Lake Allison fell. The Tualatin Ice Age Foundation and IAFI both describe this as the "crossroads" hydrology: over 40 flood cycles, water entered the Tualatin Valley through Lake Oswego and exited back into the Willamette Valley through Tualatin. The slow rise and slower fall of backflooded water in the Tualatin allowed the finest sediment to settle out, which is why the Tualatin floor today is some of the richest agricultural soil in Oregon. It is also why the Tualatin Valley holds an unusually high concentration of intact ice-rafted erratics. Boulders dropped when icebergs grounded on the rising lake bed or melted in place during the long quiet phase, and many were not re-entrained by subsequent floods because the next flood arrived as a quiet backwater rather than a scouring current. The Tualatin Heritage Center now holds several recovered erratics including a 20,000-pound granite boulder and a 5,500-pound quartzite specimen, both unearthed from local farms.
South of Tualatin, between Sherwood and Wilsonville, the Tonquin Scablands record what happened when flood water spilled over the low divide between the Tualatin and the Willamette mainstem during peak stage. Geologists have identified 14 distinct flood channels in this area along with kolk ponds, depressions scoured out by vertical-axis whirlpools, the largest of which is the 3.5-acre Coffee Lake. The Tonquin features are the only scabland-style erosional landscape inside the Portland-Tualatin region. The Willamette Meteorite, the 15.5-ton iron meteorite known to the Clackamas as Tomanowos, was rafted to West Linn on a Cordilleran-derived iceberg and dropped when the ice melted; it is the largest object known to have been delivered by the floods anywhere in the system.
J Harlen Bretz first walked the Portland Basin in the early 1920s and treated it as the terminal feature of the Spokane flood: the great gravel delta at the mouth of the Gorge was, in his framing, the depositional counterpart to the erosional Channeled Scabland upstream. Bretz mapped the gravel apron from Troutdale west into east Portland and identified it as flood-flow sediment rather than ordinary fluvial gravel based on cross-bedding scale, foreset orientation, and clast size. His Gorge work, conducted as part of the same 1922-1931 field seasons, included the cataracts at Multnomah, Latourell, Bridal Veil, Wahkeena, Horsetail, and Oneonta, all of which he argued were hanging-valley waterfalls produced when sudden over-deepening of the Columbia main stem truncated their lower courses. The basalt amphitheater at Multnomah was, for Bretz, the cleanest single demonstration that the Gorge had been excavated catastrophically rather than over millions of years.
Ira S. Allison of Oregon State extended the work south into the Willamette Valley. His 1935 Geological Society of America Bulletin paper "Glacial Erratics in the Willamette Valley" was the first systematic catalog of ice-rafted boulders in the basin. He documented "no less than 300 occurrences" of erratics in a near-continuous ring around the lower hillsides at roughly constant elevation, a pattern that demanded a former lake surface. His 1953 work then identified the Willamette Silt as a lacustrine deposit correlative with the rhythmites of eastern Washington, spanning roughly 3,000 square miles of the valley floor and reaching 180 to 200 feet thick in places. Allison did not yet have the Lake Missoula source story (Bretz's flood hypothesis was still being rehabilitated when Allison's silt paper came out), but his stratigraphic correlation between the Willamette Silt and the Lake Lewis sediments of the Pasco Basin is the link that joins the two halves of the system. Lake Allison, the name now used for the temporary lake in the Willamette Valley, was assigned in his honor.
The Portland Basin has become one of the most carefully reread reaches of the flood system in the last fifteen years. Evarts, O'Connor, Wells, and Madin's 2009 GSA Today paper "The Portland Basin: A (big) river runs through it" reframed the basin's stratigraphic architecture as a four-phase sequence: (1) deposition of a broad pre-flood Columbia gravel plain, (2) valley incision into that plain, (3) flood-driven valley scouring and side-terracing, and (4) post-flood submersion of the axial valley by sea-level rise. The flood deposits are the third phase, and they sit unconformably on the older basin fill. Subsurface borehole and cross-section work in the basin has shown that flood gravel is much more spatially restricted than the surface exposure suggests; the floods reworked only a narrow corridor down the basin axis, leaving most of the older basin fill intact beneath the Willamette Silt.
The 18.2 ka chronological anchor from Balbas, Barth, Bierman, Caffee, Rood, and Zimmerman (Geology, 2017) applies to the Portland-Tualatin region because peak Lake Allison stage in the Willamette Valley is hydraulically coupled to the Lake Lewis high stand at Wallula upstream. The same flood that filled Lake Lewis to its 1,200-foot stage at Wallula also filled Lake Allison to its 400-foot stage in the Willamette. Backwater modeling in O'Connor, Baker, Waitt, Smith, Cannon, George, and Denlinger (Earth-Science Reviews, 2020) shows that the two lakes were essentially in hydraulic communication during peak flow through the Gorge. The 2020 review collapses the previous "Missoula floods vs. Bonneville flood" distinction at this reach and treats every late Pleistocene high-stage deposit in the Portland Basin as a Missoula-source event. Recent work by David and others (Geophysical Research Letters, 2022) on paleo-canyon geometry in the Gorge has revised peak discharge estimates downward somewhat, suggesting the Gorge was narrower at the time of the floods than the modern channel suggests, which raises stage but lowers total discharge. The Willamette Silt itself, exposed in cuts and excavations from Portland south to Eugene, has been read as a stack of rhythmically graded slackwater beds equivalent to the Touchet Beds upstream, recording dozens of separate flood cycles. USGS Open-File Report 2003-408 (O'Connor, Sarna-Wojcicki, Wozniak, Polette, and Fleck) provides the canonical inundation-depth, erratic-distribution, and sedimentary-facies map for the Willamette Valley and is the most heavily used reference for the lake's geometry and the erratic catalog.
The Tualatin Ice Age Walking Trail is the most accessible interpretive site in the region. It is a self-guided urban trail with seven sub-loops, signed throughout, that ties together the Tualatin Heritage Center (8700 SW Sweek Drive), the Tualatin Greenway along the Tualatin River with its blue-mosaic concrete path representing the flood path, mastodon and giant ground sloth discovery sites within the city, and an Ice Age-themed ArtWalk. The Tualatin Heritage Center holds the largest publicly displayed flood erratics in the metro area, all plaqued, and hosts the Lower Columbia Chapter of the Ice Age Floods Institute, which meets on the third Thursday of every month except August and December. The chapter records its lectures and posts them to the IAFI YouTube channel. The Ice Age Tonquin Trail, a 22-mile multi-use trail connecting Sherwood, Tualatin, and Wilsonville, passes the Coffee Lake kolk pond and several flood channels. The Crown Point Vista House and the adjacent Portland Women's Forum State Scenic Viewpoint at Chanticleer Point are the canonical free overlooks for reading flood stage at the Gorge mouth. Rocky Butte in northeast Portland is accessible by car to a summit park with views across the basin and interpretive signage about the high stand. The Willamette Meteorite has a small interpretive site in West Linn near its original find location, though the meteorite itself is at the American Museum of Natural History in New York. OMSI in Portland does not maintain a permanent Ice Age Floods exhibit; the most recent flood-related programming was the temporary 2026 "Heads and Hearts: Seeing the Landscape through Nez Perce Eyes" exhibit, which addressed the floods through Plateau Tribes' oral history.
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.
We now know that temporary Lake Allison, which formed when floodwaters backed up into the Willamette Valley, covered roughly 3,000 square miles and deposited the fine sediments that make this some of Oregon's most fertile farmland.