The ups and downs of the Missouri River from Pleistocene to present: Impact of climatic change and forebulge migration on river profiles, river course, and valley fill complexity

The Missouri River is a continent-scale river that has thus far escaped a rigorous reporting of valley fill trends within its trunk system. This study summarizes evolution of the lower Missouri River profile from the time of outwash in the Last Glacial Maximum (LGM) until establishment of the modern dominantly precipitation-fed river. This work relies on optically stimulated luminescence (OSL) dating, water-well data, and a collection of surficial geological maps of the valley compiled from U.S. Geological Survey EDMAP and National Science Foundation Research Experience for Undergrads projects. Mapping reveals five traceable surfaces within valley fill between Yankton, South Dakota, USA, and Columbia, Missouri, USA, that record two cycles of incision and aggradation between ca. 23 ka and ca. 8 ka. The river aggraded during the LGM to form the Malta Bend surface by ca. 26 ka. The Malta Bend surface is buried and fragmented but presumed to record a braided outwash plain. The Malta Bend surface was incised up to 18 m between ca. 23 ka and ca. 16 ka to form the Carrolton surface (ca. 16 ka to ca. 14 ka). The Carrollton surface ghosts a braided outwash morphology locally through overlying mud. Aggradation followed (ca. 14 ka to ca. 13.5 ka) to within 4 m of the modern floodplain surface and generated the Salix surface (ca. 13.5 to ca. 12 ka). By Salix time, the Missouri River was no longer an outwash river and formed a single-thread meandering pattern. Reincision at ca. 12 ka followed Salix deposition to form the short-lived Vermillion surface at approximately the grade of the earlier Carrolton surface. Rapid aggradation from ca. 10 ka to ca. 8 ka followed and formed the modern Omaha surface (ca. 8 ka to Present). The higher Malta Bend and Omaha profiles are at roughly the same grade, as are the lower Carrolton and Vermillion surfaces. The Salix surface is in between. All surfaces converge downstream as they enter the narrow and shallow bedrock valley just before reaching Columbia, Missouri. The maximum departure of the profiles is 18 m near Sioux City, Iowa, USA, at ∼100 km downstream from the James Lobe glacial input near Yankton, South Dakota. Incision and aggradation appear to be driven by relative changes in input of sediment and water related to glacial advance and retreat and then later by climatic changes near the Holocene transition. The incision from the Malta Bend to the Carrolton surface records the initial breakdown of the cryosphere at the end of the LGM, and this same incisional event is found in both the Ohio and Mississippi valleys. This incisional event records a “big wash” that resulted in the evacuation of sediment from each of the major outwash rivers of North America. The direction and magnitude of incision from the LGM to the modern does not fit with modeled glacioisostatic adjustment trends for the Missouri Valley. Glaciotectonics likely influenced the magnitude of incision and aggradation secondarily but does not appear to have controlled the overall timing or magnitude of either. Glaciotectonic valley tilting during the Holocene, however, did likely cause the Holocene channel to consistently migrate away from the glacial front, which argues for a forebulge axis south of the Missouri Valley during the Holocene and, by inference, earlier. This is at least 200 km south of where models predict the Holocene forebulge axis. The Missouri Valley thus appears to reside in the tectonic low between the ice front and the forebulge crest. The buffer valley component of incision caused by profile variation could explain as much as 25 m of the total ∼40 m of valley incision at Sioux City, Iowa. The Missouri Valley also hosted a glacial lobe as far south as Sioux City, Iowa, in pre-Wisconsinan time, which is also a factor in valley excavation.