TRACING SEDIMENT SOURCE RESPONSE TO WATER INFRASTRUCTURE DEVELOPMENT AND MANAGEMENT IN MEDICINE LAKE NATIONAL WILDLIFE REFUGE, MONTANA

Medicine Lake, within the Medicine Lake National Wildlife Refuge complex in northeastern Montana, is a crucial stopover point for migratory birds traveling between wintering and breeding sites at opposite ends of the Central North American Flyway and provides nesting habitat for various waterfowl species, including the endangered piping plover. After two episodes of Medicine Lake drying out in 1900 and 1935, the Civilian Conservation Corps installed water control structures within the lake, including a large diversion channel from nearby Big Muddy Creek, in order to mitigate habitat loss during future periods of drought. Water level management practices since this time introduced an unknown amount of sediment into the lake basin, with unknown impacts to the lake ecosystem and associated wildlife. To better describe these impacts within Medicine Lake, we analyzed the magnetic and physical properties of four sediment cores that capture 20th century sedimentation within the lake. Comparison of bulk magnetic properties of sediments from sites proximal to historic water control infrastructure with more distal sites reveal differences in sediment dynamics and/or authigenic processes within the lake basin. Sites close to the lake margin and more proximal to the water control structures show considerable changes in both size and concentration of magnetic grains. These sediments are characterized by high concentrations of fine-grained magnetic minerals prior to lake desiccation and low concentrations of coarse-grained magnetic minerals in more recent sediments deposited post-management. Sediments from more distal sites are consistent with low concentrations of coarse grained magnetic minerals pre and post management. We compare the bulk magnetic properties of lake core sediments to those of surficial sediment sources surrounding the lake basin to evaluate historic fluxes in sediment sources through the twentieth century. Finally, we compare our magnetic data to physical characteristics of these lake sediments to better constrain and trace sediment sourcing within Medicine Lake.