History of the Missouri River Valley from the Late Pleistocene to Present: Climatic vs. Tectonic Forcing on Valley Architecture

Results from this study provide evidence for two cycles of aggradation and incision within the Missouri River Valley since the Last Glacial Maximum. Timing of these cycles were compared with drainage histories from deglaciation and glacial isostatic adjustment (GIA) from glacial loading and unloading. Comparison of the timing and magnitude of these cycles to modeled GIA rates and locations suggest these cycles were not caused by tectonic movement from GIA. Instead, incision and aggradation events fit better with glacial drainage patterns from the Laurentide ice sheet. This study also indicates that aggradation and incision events at the end of the LGM to 16 ka BP maybe a North American phenomenon as supported by similar responses in the Ohio and Mississippi River Valleys. While GIA does not support incisions and aggradation, this study does provide evidence for valley tilting affecting lateral migration of Missouri River Channel belts over the last 8 ka. A longitudinal profile created from Yankton, SD to Columbia, MO indicates the river valley has a higher "buffer" capacity around Elk Point, SD which thins to the north and the south. A possible "buttress" effect is indicated by very small vertical movement of river profiles in the Kansas City to Columbia, MO area. This buttress effect is most likely due to an inability for the Missouri River to incise into the narrow and shallow bedrock valley which develops around Arrow Rock, MO. Valley architectures created within the Missouri River Valley have a similar distribution of channels and floodplain fines as models previously tied to down-stream anchored base-level controls. Findings within this study provide evidence for this architecture being built by preferential preservation within a climatically driven "buffer" system by multiple aggradation and incision events.