SGER: Analysis and Paleohydrologic Application of Sedimentary Deposits of the Record Breaking 1993 Mississippi River Flood

小行星9409778 本项目的总体目标是定量分析1993年密西西比河上游和下游爱荷华州Keokuk的破纪录洪水的野外地层记录。 对Keokuk河水位计的洪水记录的分析表明，1993年的洪水在该河段的重现频率预计为500年一次。 因此，对1993年洪水造成的地层记录进行分析，提供了一个机会，可以对历史记录中很少见到的罕见洪水事件的沉积学和地球化学性质建立定量记录。 具体项目的目标包括：（1）工程调查，以定量确定沉积物沉积深度和体积与沉积表面相对于洪水主流的相对高度之间的关系;（2）分析洪水沉积单元的沉积学特性，包括粒度特征和沉积结构;（3）洪水沉积序列中有机碳和碳酸钙镁的垂向变化分析;（4）支流静水沉积分析，以评价受以下因素影响的沉积特征：支流规模和坡度、1993年密西西比洪水期间和之后的支流洪水历史以及支流与主流密西西比河谷的交汇角。 必须尽快对1993年的洪水沉积物进行分析，因为大多数位于农业耕作的土地上。 因此，在即将到来的作物生长季节开始之后，不太可能获得许多优秀的沉积层序，届时将进行春季作物种植。 研究结果将显著改进基于洪水事件地质记录的古水文学方法，因为在潮湿气候区（如以密西西比河谷上游为代表），沉积物的自然生物扰动和地球化学变化通常使地质记录中的细粒漫滩地层记录难以解释。 因此，记录新鲜的，未风化的，相对不受干扰的沉积物从罕见的，大规模的1993年洪水将是特别有益的，以提高对关键的物理证据的理解，应寻求在地质记录，重点是重建过去的历史，洪水的规模和频率。 最后，鉴于河流沉积物在地球化学循环中发挥的重要作用，研究结果也将大大有助于提高对冲积系统中长期沉积物流动性和储存的认识。

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.