9409778 Knox The general objective of this project is to quantitatively analyze the field stratigraphic record of the record-breaking 1993 flood on the middle reach of the upper Mississippi River upstream and downstream of Keokuk, Iowa. Analysis of flood records for the Keokuk River gage indicates that the 1993 flood had an expected recurrence frequency of only once in 500 years for this reach of the river. Analysis of the stratigraphic record resulting from this 1993 flood therefore provides an opportunity to establish quantitative documentation of sedimentological and geochemical properties of a rare flood event that is seldom witnessed in historical records. Project-specific objectives include (1) engineering surveys to quantitatively determine relationships between depths and volumes of sediment deposition in relation to relative heights of depositional surfaces with respect to main currents of flood flow; (2) analysis of sedimentological properties of flood depositional units, including grain size characteristics and sedimentary structures; (3) analysis of vertical variations of organic carbon and calcium and magnesium carbonate in flood depositional sequences; and (4) analysis of slackwater sedimentation in sidevalley tributaries to evaluate sedimentation characteristics as influenced by: tributary size and gradient, tributary flooding history during and since the 1993 Mississippi flood, and tributary junction angle with mainstream Mississippi Valley. Analyses of the 1993 flood deposits need to be undertaken as soon as possible because most are located on land that is under agricultural cultivation. Therefore, it is unlikely that many of the excellent depositional sequences will be available beyond the beginning of the coming growing season when cultivation for spring planting of crops will take place. Results of the search will significantly improve the methods of paleoflood hydrology which are based on the geological record of flood events bec ause natural bioturbation and geochemical alterations of sediments in humid climate regions, such as represented by the upper Mississippi Valley, normally make the fine-grained overbank stratigraphic record difficult to interpret in the geological record. Hence, documentation of fresh, unweathered, and relatively undisturbed sediments from the rare, large-magnitude 1993 flood will be particularly beneficial for improved understanding of key physical evidence that should be sought in the geologic record where emphasis is on reconstructing past histories of flood magnitudes and frequencies. Finally, given the important role that river sediments play in geochemical cycles, the results of the research will also contribute significantly to improved understanding of long-term sediment mobility and storage in alluvial systems.
