MAPPING BATHYMETRY IN A LARGE MEANDERING RIVER ABOVE AND BELOW A SIGNIFICANT SEDIMENT INPUT

Accurate representations of river bathymetry are essential for understanding channel morphodynamics and sediment routing. Repeat surveys of channel topography using sophisticated technology can elucidate where depositional surfaces occur and whether they are acting as net accumulation zones or places of transient sediment exchange. In this study, we used an Acoustic Doppler Current Profiler (ADCP) coupled with a real-time-kinematic GPS to collect bathymetry data upstream and downstream of a critical tributary junction where the Blue Earth River joins the Minnesota River, near Mankato, MN, USA. The Blue Earth River represents a considerable point source of coarse and fine sediment, much of which is stored within the channel and floodplain of the actively aggrading Minnesota River. This junction therefore represents a rare opportunity to study how a significant increase in sediment supply influences the form and evolution of in-channel topography. We surveyed 33 river km over 6 days in June 2013, achieving an average point density of 3.2 points/100 m2. River bathymetry is generally more variable in the transverse than the streamwise direction, so we used anisotropic interpolation techniques to construct digital elevation models (DEMs) of the surveyed channel. We transformed Cartesian coordinates to curvilinear orthogonal coordinates and generated DEMs using Hutchinson’s spline, anisotropic ordinary kriging, and elliptical inverse distance weighting statistical interpolation techniques. To evaluate the performance of each interpolation method we created DEMs from a subset of the surveyed points and calculated the root mean square error between reference points in each DEM. We observed alternate bars in straight reaches downstream of the tributary junction, while straight reaches upstream were planar bed. Point bars and cutoffs were observed in meandering reaches above and below the tributary junction. Channel geometry significantly differs downstream of the Blue Earth River confluence. Average channel width increases 1.5x, the width to depth ratio nearly doubles, and bed slope decreases. We will continue to survey the Minnesota River for the next three years to quantify changes in the form and organization of channel topography.