MEASURING THE IMPACT OF EXTREME FLOODS ON FLUVIAL GEOMORPHOLOGY IN WISCONSIN’S BAD RIVER WATERSHED

The Bad River (Mashkiiziibii) drains 2,512 km² in northern Wisconsin. During July 2016, several rounds of convective storms affected the region, generating rainfall totals between 20 and 25 centimeters within an 8-hour span. This intense precipitation led to an extreme flood on the Bad River (annual exceedance probability < 0.2%), closing transportation corridors and causing more than $25 million USD in infrastructure damages regionally. Significant flooding also occurred in 2018. Much of the damage to infrastructure was associated with fluvial erosion and deposition in proximity to river and stream crossings. To identify areas of significant fluvial change, we generated seamless pre- and post-flood 1-meter LiDAR-derived digital elevation models for the entire Bad River watershed and subtracted the surfaces to generate an elevation change raster. We used semi-automated methods to identify channel banks and extract river segment attributes including channel slope, valley slope, proximity to steep hillsides, and surficial geology, among others. Our analysis revealed thousands of significant elevation change features associated with point bar and floodplain accretion, bank erosion, and mass wasting of river bluffs. We found that post-glacial setting impacted erosion and deposition patterns, with stream segments in steep terrain and heterogeneous surficial lithology associated with paleoshorelines exhibiting a higher density of fluvial change features in comparison to areas of the watershed underlain by bedrock or homogeneous lacustrine clays. Understanding the fluvial geomorphic changes associated with extreme floods is crucial for identifying landscape factors that predict fluvial instability, ultimately aiding management organizations in reducing exposure to fluvial hazards.