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

9409778诺克斯该项目的总体目标是对密西西比河上游、爱荷华州基奥库克上游和下游1993年创纪录的洪水的现场地层记录进行定量分析。对基奥库克河洪水记录的分析表明，1993年的洪水预计在该河段500年才会再次发生一次。因此，对1993年洪灾所产生的地层记录的分析提供了一个机会，对历史记录中很少见到的一次罕见的洪灾事件的沉积学和地球化学性质建立定量记录。具体项目目标包括：(1)工程勘测，以定量确定沉积面与主流高度之间的沉积深度和体积之间的关系；(2)分析洪水沉积单元的沉积学性质，包括粒度特征和沉积结构；(3)分析洪水沉积序列中有机碳、钙和碳酸镁的垂直变化；(4)分析侧谷支流的死水沉积，以评估下列因素对沉积特征的影响：支流的大小和坡度、1993年密西西比洪水期间及以来的支流洪水历史、与密西西比主流河谷的支流交汇角度。需要尽快对1993年的洪水沉积物进行分析，因为大多数都位于农业耕种的土地上。因此，许多优良的沉积层序不太可能在即将到来的生长季节开始后才能获得，届时将进行春季作物种植。由于以密西西比河谷上游为代表的湿润气候区沉积物的自然生物扰动和地球化学变化，通常使细粒的滨岸地层记录在地质记录中难以解释，因此研究成果将极大地改进基于洪水事件地质记录的古洪水水文方法。因此，对1993年罕见的大洪水的新鲜、未风化和相对未受干扰的沉积物进行记录，将特别有利于更好地理解应在地质记录中寻找的关键物理证据，因为地质记录的重点是重建过去的洪水规模和频率历史。最后，鉴于河流沉积物在地球化学循环中发挥的重要作用，研究结果还将大大有助于更好地了解冲积体系中沉积物的长期迁移和储存。

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.