LONG-TERM GEOMORPHIC IMPACTS OF LONGITUDINAL TRAINING WALLS IN THE GRAND RIVER, MICHIGAN (USA)

The Grand River is the longest river in Michigan, flowing from northwest Hillsdale County to Lake Michigan, and draining an area over 14,400 square kilometers. Natural rapids roughly 1-mile long have been submerged since ~1835 as a result of dam construction in the city of Grand Rapids. Bathymetric surveys of a ~38 km reach of the river from Grand Rapids to the confluence with Bass River, performed in 1906, were uncovered in the Grand Rapids Museum archives. These maps show depth recordings across the channel along cross sections spaced ~90 meters apart. The maps also show the location of numerous training walls that were built in the river in the 1890’s as an effort to maintain navigation. The 1906 maps were georeferenced, and the bathymetric data were digitized, converted to bed elevation, and interpolated into an elevation model of the channel bed. Detailed comparisons and difference maps between the 1906 survey and multibeam echo sounding surveys conducted in June 2019 and 2020 over the same reach provide a means to assess over 100-years of geomorphic changes including volume difference, width, longitudinal profile and cross sectional profile changes and response to human activities. Results show many of the training walls are still present and have had a significant impact on the geomorphology of the river, resulting in the development of many mid-channel islands and narrowing of the channel. Findings from this study have substantial implications for dam removal and dredging projects that have been proposed for the river.