Wetland hydrology and the impacts of beaver dams in the Upper Columbia River floodplain wetlands

Abstract

The Upper Columbia River floodplain wetlands are the last remaining undammed stretch of floodplain wetlands along the Columbia River and continue to experience a natural flood pulse. This flood pulse interacts with natural levees and beaver dams across the floodplain and habitat heterogeneity results, with individual wetlands within the Columbia Wetlands having different hydrographs. I conducted research in 38 wetlands from 2020 to 2022 and aimed to determine if differing wetland hydrology allowed for wetland groups to be determined, and if those groups could be attributed to gaps in natural levees and beaver dams. Hydrograph attributes can be used to differentiate wetland groups, with three or four groups being identified depending on year. Random Forest models based on measurements of the levees, levee gaps, and beaver dams had an Out-Of-Box Error Estimate of between 36% and 53% across all groups depending on year, indicating correct classification of between 64% and 47%. Combining hydrograph attributes and levee gap and beaver dam metrics, we can describe these groups on a gradient of connectivity to the Columbia River, being Most Connected, with large open levee gaps, Partially Connected, with levees without gaps or with gaps dammed by beaver dams that are smaller, and Least Connected, with levees without gaps or gaps dammed by beaver dams that are bigger. This demonstrates the large impacts of beavers on shaping wetland systems and has implications for the differing impacts of climate change on these different wetland groups.