Journal of Hydrology, 40(1979)85--99 85 ~-3 Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands [2] ALLUVIAL HYDROGEOLOGY OF THE LOWER MISSOURI RIVER VALLEY NORMAN G. GRANNEMANN and JOHN M. SHARP, Jr. Geology Department, University of Missouri, Columbia, MO 65201 (U.S.A.) (Received November 15, 1977; revised and accepted June 25, 1978) ABSTRACT Grannemann, N.G. and Sharp, Jr., J.M., 1979. Alluvial hydrogeology of the lower Missouri River valley. J. Hydrol., 40: 85--99. Floodplain deposits in the lower Missouri River valley alluvium can be divided horizontal- ly 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 conduc tivity 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 vari- ation. 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, flood- plain inhomogeneities, tributary creeks, and pumping may cause perturbations in ground- water levels. INTRODUCTION Extensive floodplain alluvium in the lower Missouri River valley contains a large relatively untapped source of groundwater. This is typical of many modern major river valleys whose active floodplains are an important agricul- tural resource due to their high natural fertility and their ideal locations for supplemental irrigation systems. Floodplains are also extensively used for re- creation and provide excellent wildlife habitat. Such alluvial systems are under- going increasing stress. Funk and Robinson (1974), for instance, pointed out that the Missouri River valley within the boundaries of the state of Missouri has lost 50% of its surface water area and virtually all of its islands. Extensive fish and wildlife habitat has been eliminated in these last 50 years. A large por- tion [greater than 12% (Sewell, 1969)] of our population lives on land subject