ALLUVIAL HYDROGEOLOGY OF THE LOWER MISSOURI RIVER VALLEY

Floodplain deposits in the lower Missouri River valley alluvium can be divided horizontally into a channel belt, meander belt and flood basin; and vertically they consist of a top-stratum and a substratum. The Missouri, like most major rivers in the midwestern U.S., is an underfit stream which does not fully penetrate its alluvium. Large amounts of groundwater are stored in these alluvial deposits, in which sediment mean grain size and hydraulic conductivity both increase exponentially with depth. Digital computer models indicate, however, that only one flow system exists in the alluvium. Digital simulations and field observations of the floodplain near Glasgow, Missouri, show four types of groundwater response to river stage changes: areas of rapidly fluctuating groundwater levels, areas of slow response, areas of predominantly down-valley flow, and areas of continuing groundwater high. The bank storage-river interaction can be mainly effluent, mainly influent, or a combination of both. The most important factor controlling the floodplain groundwater system is river stage variation. Distance from the river, the time interval since the river has fallen or risen, and the geometry of the river and valley walls all affect the groundwater system. In addition, floodplain inhomogeneities, tributary creeks, and pumping may cause perturbations in groundwater levels. © 1979.