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.

ABSTRACT In late Wisconsin time, the Purcell Trench lobe of the Cordilleran ice sheet dammed the Clark Fork of the Columbia River in western Montana, creating glacial Lake Missoula. During part of this epoch, the Okanogan lobe also dammed the Columbia River downstream, creating glacial Lake Columbia in northeast Washington. Repeated failure of the Purcell Trench ice dam released glacial Lake Missoula, causing dozens of catastrophic floods in eastern Washington that can be distinguished by the geologic record they left behind. These floods removed tens of meters of pale loess from dark basalt substrate, forming scars along flowpaths visible from space. Different positions of the Okanogan lobe are required for modeled Missoula floods to inundate the diverse channels that show field evidence for flooding, as shown by accurate dam-break flood modeling using a roughly 185 m digital terrain model of existing topography (with control points dynamically varied using automatic mesh refinement). The maximum extent of the Okanogan lobe, which blocked inundation of the upper Grand Coulee and the Columbia River valley, is required to flood all channels in the Telford scablands and to produce highest flood stages in Pasco Basin. Alternatively, the Columbia River valley must have been open and the upper Grand Coulee blocked to nearly match evidence for high water on Pangborn bar near Wenatchee, Washington, and to flood Quincy Basin from the west. Finally, if the Columbia River valley and upper Grand Coulee were both open, Quincy Basin would have flooded from the northeast. In all these scenarios, the discrepancy between modeled flood stages and field evidence for maximum flood stages increases in all channels downstream, from Spokane to Umatilla Basin. The pattern of discrepancies indicates that bulking of floods by loess increased flow volume across the scablands, but this alone does not explain low ­modeled flow stages along the Columbia River valley near Wenatchee. This latter discrepancy between modeled flood stages and field data requires either additional bulking of flow by sediment along the Columbia reach downstream of glacial Lake Columbia, or coincident dam failures of glacial Lake Columbia and glacial Lake Missoula.

Abstract The Channeled Scabland of east-central Washington comprises a complex of anastomosing fluvial channels that were eroded by Pleistocene megaflooding into the basalt bedrock and overlying sediments of the Columbia Plateau and Columbia Basin regions of eastern Washington State, U.S.A. The cataclysmic flooding produced huge coulees (dry river courses), cataracts, streamlined loess hills, rock basins, butte-and-basin scabland, potholes, inner channels, broad gravel deposits, and immense gravel bars. Giant current ripples (fluvial dunes) developed in the coarse gravel bedload. In the 1920s, J Harlen Bretz established the cataclysmic flooding origin for the Channeled Scabland, and Joseph Thomas Pardee subsequently demonstrated that the megaflooding derived from the margins of the Cordilleran Ice Sheet, notably from ice-dammed glacial Lake Missoula, which had formed in western Montana and northern Idaho. More recent research, to be discussed on this field trip, has revealed the complexity of megaflooding and the details of its history. To understand the scabland one has to throw away textbook treatments of river work. —J. Hoover Mackin, as quoted in Bretz et al. (1956, p. 960)

Abstract This one-day field trip of geologic and historical significance goes from Washtucna, Washington, through Palouse Falls State Park, Lyons Ferry State Park, and Starbuck, and ends in the Tucannon River valley. At Palouse Falls, it is readily apparent why Native Americans crafted stories about the origins of this spectacular area and why geologic debates regarding the role of Pleistocene glacial Lake Missoula floods during the formation of this natural wonderland have been centered here. This field trip focuses on structural geology and the Palouse Falls fracture zone, Columbia River Basalt Group stratigraphy at the falls, and subsequent erosion by glacial outburst floods. Discussion of the falls will include human history and the formation of Palouse Falls State Park. The main stop at Palouse Falls will explore the stratigraphy of the Columbia River Basalt Group, Vantage Member, loess islands, fracture zones, and human history dating back at least 12,000 yr. Driving south through Lyons Ferry State Park and the Tucannon Valley, we will discuss topics ranging from the Palouse Indians to sheep herding and from clastic dikes to terracettes.

Abstract Glacial Lake Missoula was repeatedly dammed by the Purcell Trench Lobe of the Cordilleran ice sheet during the last glaciation to maximum altitudes near 4200 ft (1280 m). Studies from outside of the lake basin suggest that the lake filled and drained multiple times in the late Pleistocene. Deposits and landforms within the former glacial lake basin provide evidence for a complex lake-level history that is not well understood for this famous impoundment. At least two general lake phases are evident in the stratigraphy: an earlier phase of catastrophic drainage that was responsible for large-scale dramatic erosional and depositional features, and a later, less-catastrophic, phase responsible for the preservation of fine-grained glaciolacustrine sediments. Features of the earlier lake phase include giant gravel dunes and openwork gravel with anomalously large clasts (erratics). Deposits from the later phase are mostly low-energy glaciolacustrine sediments that record a history of lake-bottom sedimentation and repeated lake-floor exposure. A focus of this field trip is to look at evidence for the two lake phases as well as evaluate the record of exposure surfaces, and therefore lake-level lowerings, during the second phase at multiple locations in the lake basin. One of the second phase sites is close to a highstand, full basin position in the lake (near Garden Gulch), representing a maximum water depth at this site of ~100 m, whereas others (Rail line and Ninemile) are at lower altitudes in regions that may have been under as much as 300 m of water. Fine-grained glaciolacustrine sediments are rippled very fine sandy silt and fining-upward sequences of laminated silt and clayey silt of glaciolacustrine origin. Periglacial features, contorted bedding, desiccation, and paleosols in outcrop provide clear evidence of multiple exposure surfaces; each represent a lake-lowering event. Optically stimulated luminescence (OSL or “optical dating”) ages on quartz from the three sections (Ninemile, Rail line, and Garden Gulch) allow for preliminary correlations that suggest approximately the same phase of glacial Lake Missoula sedimentation. The exposure surfaces suggest that the glacial-lake level rose and fell at least 8–12 times to elevations above and below the sections (936–1180 m), filling to within 100 m of full pool (1280 m). Optical dating shows that this occurred after 20 ka and the last inundation of the lake before 13.5 ka. Correlation of specific exposure surfaces throughout the basin will be required to develop a lake-level history.

The Columbia River system is one of the great river systems of North America, draining much of the Pacific Northwest, as well as parts of the western United States and British Columbia. The river system has had a long and complex history, slowly evolving over the past 17 m.y. The Columbia River and its tributaries have been shaped by flood basalt volcanism, Cascade volcanism, regional tectonism, and finally outburst floods from Glacial Lake Missoula. The most complex part of river development has been in the northern part, the Columbia Basin, where the Columbia River and its tributaries were controlled by a subsiding Columbia Basin with subtle anticlinal ridges and synclinal valleys superimposed on a flood basalt landscape. After negotiating this landscape, the course to the Pacific Ocean led through the Cascade Range via the Columbia Trans-Arc Lowland, an ancient crustal weakness zone that separates Washington and Oregon. The peak of flood basalt volcanism obliterated the river paths, but as flood basalt volcanism waned, the rivers were able to establish courses within the growing fold belt. As the folds grew larger, the major pathways of the rivers moved toward the center of the Columbia Basin where subsidence was greatest. The finishing touches to the river system, however, were added during the Pleistocene by the Missoula floods, which caused local repositioning of river channels.

Abstract Landslides and floods of lava and water tremendously affected the Columbia River during its long history of transecting the Cascade Volcanic Arc. This field trip touches on aspects of the resulting geology of the scenic Columbia River Gorge, including the river-blocking Bonneville landslide of ~550 years ago and the great late-Pleistocene Missoula floods. Not only did these events create great landscapes, but they inspired great geologists. Mid-nineteenth century observations of the Columbia River and Pacific Northwest by James Dwight Dana and John Strong Newberry helped germinate the “school of fluvial” erosion later expanded upon by the southwestern United States topographic and geologic surveys. Later work on features related to the Missoula floods framed the career of J Harlen Bretz in one of the great geologic controversies of the twentieth century.

ABSTRACT The late Wisconsin Missoula floods are Earth's largest known discharges of fresh water. They carved Washington's Channeled Scabland—made famous by J.H. Bretz's writings in the 1920s to 1950s—and deposited sporadic huge gravel bars in the Scab-lands and Columbia valley. Since the late 1970s the great floods have been shown to number several score and to have been released as gigantic jökulhlaups. This five-day fieldtrip zig-zags broadly along and across the Scablands and down Columbia valley, viewing much geomorphic and stratigraphic evidence of the Missoula floods, at the end washing into Portland and Geological Society of America's 2009 Annual Meeting.

ABSTRACT The Columbia River Basin (CRB) is home to the best studied examples of two of the most spectacular geologic processes on Earth and Mars: flood volcanism and catastrophic water floods. Additionally, features formed by a variety of eolian, glacial, tectonic, and mass-wasting processes can also be seen in the CRB. These terrains provide exceptional terrestrial analogs for the study of similar processes on Mars. This field guide describes four one-day trips out of Moses Lake, Washington, to observe a wide variety of Mars analogs.