NEW FINDINGS ON THE QUATERNARY GEOLOGIC HISTORY OF THE LOWER COLUMBIA RIVER CORRIDOR

Ongoing mapping and geochronology are redefining the geologic history and distribution of widespread Quaternary deposits in the lower Columbia River corridor between Bonneville Dam and the Cowlitz River confluence, including the Portland Basin. We are finding evidence that this region underwent episodes of substantial incision and aggradation over the last ~1 million years, spanning a vertical range of ~220 m from more than 80 m below present sea level to about 140 m above. This incision and aggradation has been driven by ~100 m amplitude changes in sea level owing to glaciation cycles, changes in regional base level due to glaciofluvial deposition at the mouth of the Lewis River valley, and episodes of Columbia River channel aggradation resulting from volcanogenic sedimentation from Mount Hood and Mount St. Helens. Localized erosional and depositional modifications to topography throughout the area also resulted from the late Pleistocene Missoula Floods, which carved flood channels and deposited gravel, sand, and silt at elevations up to 120 m above sea level. The modern Columbia River floodplain has been aggrading for the last 12,000 years since the last sea-level lowstand, and much of its present morphology downstream of the Columbia River Gorge has resulted from large inputs of volcanogenic sediment via the Sandy River from Mount Hood 1500 and 200 years ago, and via the Lewis, Kalama, and Cowlitz Rivers from Mount St. Helens 2500 and 500 years ago.