NEW VIEW OF MISSOULA MEGAFLOOD PATTERNS IN UPPER GRAND COULEE

Upper Grand Coulee is the biggest single conduit of Earth’s largest known freshwater discharges. Tributaries on both sides of the coulee above the level of Missoula-floodswept scablands reveal a preglacial “Coulee creek” along a former high-level vale about where catastrophic floods later gouged out the deep and wide coulee. This preglacial vale along a shallow structural trough between northeast-trending anticlines warping the Miocene Columbia River basalt became a natural overflow route for Pleistocene glacial Lake Missoula megafloods. These floods, running down Columbia valley, hydraulically ponded at the sharp valley bend and narrows now utilized for Grand Coulee Dam. A recent geographic and geomorphic analysis (Waitt, Chap 18 in GSA Special Paper 548 [2021]) shows that the northeast end of upper Grand Coulee had probably cut through into Columbia valley before late Wisconsin time.

Odd scabland elements and certain enigmatic late-Wisconsin deposits stem from Missoula floods steered along the changing margin of late Wisconsin Cordilleran ice during its maximum stand in the coulee. A maximum ice stand damming the entire coulee width near Northrup Canyon including the canyon’s mouth would confine Missoula floods along this reach of the coulee to upland basalt bedrock, isolating the floods from sub-basalt granodiorite that floors upper Grand Coulee and Northrup Canyon. This glacial pattern permits the granodiorite-free, wholly basaltic composition of a huge late Wisconsin flood-gravel bar built into Hartline basin at the downstream end of upper Grand Coulee. Retreat of the ice lobe by only a kilometer unblocked the mouth of Northrup Canyon then allowed the canyon to function as a cataract during later Missoula floods. Further ice retreat uncovered widespread knolls of granodiorite on the northern coulee floor, the source of abundant crystalline boulders among younger Missoula flood bars.

New 2-D hydraulic modeling of the Scabland floods (Denlinger et al., Chap 17 in GSA Special Paper 548) supports hypothesized megaflooding patterns under Pleistocene ponding scenarios caused by changing configurations of the Cordilleran ice margin.