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Abstract The Channeled Scabland of eastern Washington (USA) was formed by outburst floods from glacial Lake Missoula. Despite chronological advances, the timing of erosion in the main flood channels is unresolved. In particular, it is still uncertain whether upper Grand Coulee, the largest canyon in the Channeled Scabland, was incised during or prior to the last glaciation. We report 10Be exposure ages from erratics in upper Grand Coulee, glacial Lake Columbia, and surrounding flood routes. Flood-transported boulders on the high-elevation east rim of Grand Coulee date to ca. 17–15 ka. Ages from boulders on the floor of Grand Coulee indicate later flooding at ca. 14 ka, which post-dated canyon incision and occurred after inundation of the Telford-Crab Creek scabland at ca. 15–14.5 ka. Prior hydraulic modeling and dating suggest the entrance to Grand Coulee was blocked by rock and that canyon incision was incomplete at ca. 17 ka; hence, we interpret the 17–15 ka exposure ages on the east rim to coincide with flow over a retreating cataract during canyon incision. Our results indicate incision of Grand Coulee was completed between 17 ka and 14 ka. The short duration of canyon incision suggests that glacial Lake Missoula generated some of the most erosive outburst floods in Earth's history. 

Abstract Pleistocene outburst floods from the drainage of glacial Lake Missoula carved bedrock canyons into the Columbia Plateau in eastern Washington, USA, forming the Channeled Scabland. However, rates of bedrock incision by outburst floods are largely unconstrained, which hinders the ability to link flood hydrology with landscape evolution in the Channeled Scabland and other flood-carved landscapes. We used long profiles of hanging tributaries to reconstruct the pre-flood topography of the two largest Channeled Scabland canyons, upper Grand Coulee and Moses Coulee, and a smaller flood-eroded channel, Wilson Creek. The topographic reconstruction indicates floods eroded 67.8 km3, 14.5 km3, and 1.6 km3 of rock from upper Grand Coulee, Moses Coulee, and Wilson Creek, respectively, which corresponds to an average incision depth of 169 m, 56 m, and 10 m in each flood route. We simulated flood discharge over the reconstructed, pre-flood topography and found that high-water evidence was emplaced in each of these channels by flow discharges of 3.1 × 106 m3 s−1, 0.65–0.9 × 106 m3 s−1, and 0.65–0.9 × 106 m3 s−1, respectively. These discharges are a fraction of those predicted under the assumption that post-flood topography was filled to high-water marks for Grand and Moses Coulees. However, both methods yield similar results for Wilson Creek, where there was less erosion. Sediment transport rates based on these discharges imply that the largest canyons could have formed in only about six or fewer floods, based on the time required to transport the eroded rock from each canyon, with associated rates of knickpoint propagation on the order of several km per day. Overall, our results indicate that a small number of outburst floods, with discharges much lower than commonly assumed, can cause extensive erosion and canyon formation in fractured bedrock. 

Significance The glacial Lake Missoula outburst floods are among the largest known floods on Earth. Dozens of these floods scoured the landscapes of eastern Washington during the last Ice Age, from 18 to 15.5 thousand years ago, forming what is known as the Channeled Scabland. We explored how changes in topography due to the solid Earth’s response to ice sheet loading and unloading influenced the history of megaflood routing over the Channeled Scabland. We found that deformation of Earth’s crust played an important role in directing the erosion of the Channeled Scabland. 

Abstract Catastrophic drainage of glacial Lake Missoula through the Columbia River Gorge, USA, produced some of the largest floods ever known. However, erosion of the gorge during flooding has not been quantified, hindering discharge reconstructions and our understanding of landscape change by megafloods. Using a neural network and geomorphic observations, we reconstructed the gorge topography and found ∼7.4 km³of rock was eroded from gorge walls. Accounting for a narrower canyon and matching flood high‐water marks resulted in peak‐flood discharge reconstructions of 6 × 10⁶–7 × 10⁶ m³ s⁻¹, which are 30%–40% lower than prior estimates based on the present‐day topography. Sediment transport modeling indicated that more frequent intermediate‐sized floods transported most of the eroded rock. Thus, similar to alluvial rivers, discharge magnitude‐frequency tradeoffs may also govern canyon formation by repeated megafloods. 

Abstract Bedrock erosion and canyon formation during extreme floods have dramatically altered landscapes on Earth and Mars. Grand Coulee was carved by outburst floods from Pleistocene glacial Lake Missoula and is the largest canyon in the Channeled Scabland, a megaflood‐scoured landscape in the northwestern USA. Quantifying paleo‐discharge is required to understand how landscapes evolve in response to extreme events, but there are few constraints on the magnitude of the floods that incised Grand Coulee; hence, we used hydraulic modeling and geologic evidence to quantify paleo‐flood discharges during different phases of canyon incision. When upper Grand Coulee was incising by headward waterfall retreat, the paleo‐discharge was 2.6 × 10⁶ m³s⁻¹, which produced shear stresses great enough to cause the waterfall to retreat via toppling of basalt columns. The largest possible flood through upper Grand Coulee, a Missoula flood which raised glacial Lake Columbia to a stage of 750 m, produced a modeled discharge of 7.6 × 10⁶ m³s⁻¹. The discharges associated with waterfall retreat and drainage of glacial Lake Columbia are >80% and ∼50% lower, respectively, than the 14–17 × 10⁶ m³s⁻¹discharge predicted by assuming the present‐day topography was inundated to the elevation of high‐water marks. Due to bedrock incision, high‐water marks may overestimate paleo‐flow depth in canyons carved by floods, hence bedrock erosion should be considered when estimating paleo‐discharge in flood‐carved canyons. Our results indicate that outburst floods with discharges and flow depths much lower than those required to inundate high‐water marks are capable of carving deep canyons. 

ABSTRACT New findings about old puzzles occasion rethinking of the Grand Coulee, greatest of the scabland channels. Those puzzles begin with antecedents of current upper Grand Coulee. By a recent interpretation, the upper coulee exploited the former high-level valley of a preflood trunk stream that had drained to the southwest beside and across Coulee anticline or monocline. In any case, a constriction and sharp bend in nearby Columbia valley steered Missoula floods this direction. Completion of upper Grand Coulee by megaflood erosion captured flood drainage that would otherwise have continued to enlarge Moses Coulee. Upstream in the Sanpoil valley, deposits and shorelines of last-glacial Lake Columbia varied with the lake’s Grand Coulee outlet while also recording scores of Missoula floods. The Sanpoil evidence implies that upper Grand Coulee had approached its present intake depth early the last glaciation at latest, or more simply during a prior glaciation. An upper part of the Sanpoil section provides varve counts between the last tens of Missoula floods in a stratigraphic sequence that may now be linked to flood rhythmites of southern Washington by a set-S tephra from Mount St. Helens. On the floor of upper Grand Coulee itself, recently found striated rock and lodgement till confirm the long-held view, which Bretz and Flint had shared, that cutting of upper Grand Coulee preceded its last-glacial occupation by the Okanogan ice lobe. A dozen or more late Missoula floods registered as sand and silt in the lee of Steamboat Rock. Some of this field evidence about upper Grand Coulee may conflict with results of recent two-dimensional simulations for a maximum Lake Missoula. In these simulations only a barrier high above the present coulee intake enables floods to approach high-water marks near Wenatchee that predate stable blockage of Columbia valley by the Okanogan lobe. Above the walls of upper Grand Coulee, scabland limits provide high-water targets for two-dimensional simulations of watery floods. The recent models sharpen focus on water sources, prior coulee incision, and coulee’s occupation by the Okanogan ice lobe. Field reappraisal continues downstream from Grand Coulee on Ephrata fan. There, some of the floods exiting lower Grand Coulee had bulked up with fine sediment from glacial Lake Columbia, upper coulee till, and a lower coulee lake that the fan itself impounded. Floods thus of debris-flow consistency carried outsize boulders previously thought transported by watery floods. Below Ephrata fan, a backflooded reach of Columbia valley received Grand Coulee outflow of small, late Missoula floods. These late floods can—by varve counts in post-S-ash deposits of Sanpoil valley—be clocked now as a decade or less apart. Still farther downstream, Columbia River gorge choked the largest Missoula floods, passing peak discharge only one-third to one-half that released by the breached Lake Missoula ice dam. 

Cosmogenic nuclide surface exposure dating and erosion rate measurements in basaltic landscapes rely primarily on measurement of 3He in olivine or pyroxene. However, geochemical investigations using 3He have been impossible in the substantial fraction of basalts that lack separable olivine or pyroxene crystals, or where such crystals were present, but have been chemically weathered. Fine-textured basalts often contain small grains of ilmenite, a weathering-resistant mineral that is a target for cosmogenic 3He production with good He retention and straightforward mineral separation, but with a poorly constrained production rate. Here we empirically calibrate the cosmogenic 3He production rate in ilmenite by measuring 3He concentrations in basalts with fine-grained (~20 lm cross-section) ilmenite and co-existing pyroxene or olivine from the Columbia River and Snake River Plain basalt provinces in the western United States. The concentration ratio of ilmenite to pyroxene and olivine is 0.78 ± 0.02, yielding an apparent cosmogenic 3He production rate of 93.6 ± 7.7 atom g-1 yr-1 that is 20–30% greater than expected from prior theoretical and empirical estimates for compositionally similar minerals. The production rate discrepancy arises from the high energy with which cosmic ray spallation reactions emit tritium and 3He and the associated long stopping distances that cause them to redistribute within a rock. Fine-grained phases with low cosmogenic 3He production rates, like ilmenite, will have anomalously high production rates owing to net implantation of 3He from the surrounding, higher 3He production rate, matrix. Semi-quantitative modeling indicates implantation of spallation 3He increases with decreasing ilmenite grain size, leading to production rates that exceed those in a large grain by ~10% when grain radii are <150 lm. The modeling predicts that for the ilmenite grain size in our samples, implantation causes production rates to be ~20% greater than expected for a large grain, and within uncertainty resolves the discrepancy between our calibrated production rate, theory, and rates from previous work. The redistribution effect is maximized when the host rock and crystals differ substantially in mean atomic number, as they do between whole-rock basalt and ilmenite.