THE WAY IT ALL BECAME THE BRAIDY BUNCH: CLIMATE CUES IN MISSOURI VALLEY ALLOSTRATA

Large continental fluvial systems record climatic alterations as either aggradational or degradational phases and/or changes in river pattern. Differences in sediment input and discharge drive these changes. Tectonic deformation along the valley floor of the Missouri Valley within the study area is negligible during the Holocene; ergo, changes in river pattern must be a reflection of climate. Allostratigraphic mapping conducted during the summer of 2009 along the Missouri River floodplain, from South Sioux City, NE to Onawa, IA, identified several features indicative of climatic shifts in the Holocene deposits of the Missouri River. Six mapping groups constructed surficial geologic maps based on hand auger cores, placed according to modern and historic aerial photos, as well as soil maps. A widespread terrace within the map sections spans the description of the Missouri River noted by the 1804 Lewis and Clark expedition. This incision event likely corresponds to the Little Ice Age; however, the exact timing of the incision is unclear. Mapping efforts also documented the transitional phase of the Missouri from a meandering to a braided system; the timing of this transition is also unknown, but well predates the incisional phase. This pattern shift might record a decrease in discharge relative to sediment supply that resembles a similar shift known from farther downstream that initiated approximately 3 kbp. Samples collected for OSL dating will constrain the timing of these alternations in river behavior in addition to meander propagation rates. Furthermore, a number of meander cut-offs discovered during the project possess an echelon arrangement along the reaches of the study area. The limbs of most of these meanders curl back towards the bedrock to the west of the map sections. This meandering pattern could potentially record a climate influence, but is currently best described as a bedrock response.