Abstract
Meander lateral migration narrows the pointbar neck and lengthens the meander thalweg, and after cutoff and oxbow lake formation the meander evolution sequence starts again. Theoretical models of meander evolution predict hydraulic gradients across the neck steepen prior to cutoff, which causes spatial and temporal intensification of intra-meander hyporheic flux and regulates important ecological functions. Detailed observations of meander evolution are needed to test these model predictions and advance our understanding of the key hydrological, hydraulic, and ecological issues associated with meander bends. However, challenges in field and laboratory experiments have prohibited such observations. This dissertation used innovative laboratory methods, computer model simulations, and the application of supplemental techniques and equipment to generate and record specific meander evolution sequences and their associated surface water and groundwater dynamics. The laboratory and model data confirmed that lateral hyporheic flux through the intra-meander zone intensifies with meander evolution, spatially from the meander apex to the meander neck and temporally from younger meander to older meander. The intensification was driven by increased head loss and decreased planimetric distance across the meander neck transect. Lagging temporal patterns of surface water head loss compared with groundwater head loss suggest these processes are partially uncoupled. Groundwater does contribute to the meander evolution sequence, including the presence of erosive seepage fluxes that accelerated neck cutoff. This dissertation research developed a new close range photogrammetry technique to measure water surface elevations in a flowing river by seeding the river with wax powder. The dissertation also determined important limitations to three MODFLOW groundwater model packages for simulating river meander hyporheic exchange. The separate papers in this dissertation all contribute to advancing our tools to study meander evolution and its influence on the ecologically important issue of hyporheic exchange.