Ground water interaction with the Missouri River at Big Muddy National Fish and Wildlife Refuge

Ground water plays an important role in the hydrologic budget of and chemical composition of many surface-water bodies. Many studies have investigated interactions between lake water and ground water and between river water and ground water. At the Overton Bottoms North Unit of the Big Muddy National Fish and Wildlife Refuge, interactions among scour hole (lake) water, river water, and ground water are of interest as the scour holes serve as habitat for protected turtles and migratory wildlife. Developing an understanding of the sources of water to these water bodies, allows refuge managers to make better informed habitat restoration decisions. This information will allow them to predict the hydrogeologic outcome when designing projects to create ephemeral and permanent water bodies for a host of biota.; The objectives were to test the physical model of the river-ground-scour hole water using the isotopic composition of the water bodies as a conservative tracer coupled with numerical modeling. Based on monthly sampling the composition of Missouri River water was distinct from that of the ground water. The compositions of the water in the scour holes should reflect the relative inputs from river water and ground water. The average delta18O compositions of scour holes were -4.3 +/- 1.1‰ and -0.9 +/- 0.9‰ which reflect a ground water source that was enriched by evaporation. Major results from modeling were that river stage could affect ground water flow direction and indirectly affect flow through the scour holes. Also evaporation accounted for 21 % and 17.6% of the hydrologic budget of the two scour holes.; The numerical modeling illustrated river stage influence on ground water flow direction and the role of evaporation in the scour holes. Modeling results are supported by the field data. Thus refuge managers need to consider changes in river stage, ground water flow direction, and intensity of evaporation when designing habitat restoration projects.