River-Floodplain Connectivity and Sediment Transport Potential: Applications to Sediment Dynamics on Floodplains and Juvenile Freshwater Mussel Settling in Rivers

River-floodplain connectivity is the degree of water-driven transport of matter, energy, and organisms between rivers and their floodplains. Recent advancement of high-resolution lidar data and numerical modeling is helpful to explore river-floodplain connectivity precisely to improve our predictions of sediment transport and deposition on floodplains. In the present work, we studied floodplain sediment transport and deposition, and juvenile mussel settling in three river systems in the US. A two-dimensional hydrodynamic model was developed and simulated model results were coupled with field measurements to study river-floodplain systems of the East Fork White River in Indiana, South River in Virginia, and Dan River in North Carolina. Results show that the East Fork White River in Indiana is capable of supplying sand to the channels on the floodplain and these floodplain channels can transport sand in suspension and gravel as bedload. These floodplain channels are supply limited under the current hydrologic regime and identified as net erosional. On the South River floodplain in Virginia, incorporating hydrologic flowpaths as an explicit measure of river-floodplain connectivity can improve predictions of floodplain sediment deposition. Three regression models were developed incorporating flow pathways and the best model was applied to hydrodynamic model results to create a spatial map of floodplain sedimentation rate. The deposition map highlights how floodplain topography and river-floodplain connectivity affect sedimentation rates and can help inform the development of floodplain sediment budgets. Lastly, streamflow conditions were investigated in the Dan River, North Carolina as they affect juvenile freshwater mussel settling. Two uplooking velocity sensors on the river bed were deployed and hydraulic parameters were measured for a 7-mo period in May-November 2019 to estimate the juvenile mussel settling. Results show that juvenile freshwater mussels as large as 280-508 µm could always be suspended during our study period and not be able to settle onto the river bed at the location of our velocity sensors. Therefore, the flow and shear velocity during our study period was high enough to prohibit the recruitment of juvenile freshwater mussels from settling out of suspension at the sensor locations. Modest flow obstructions such as large boulders, downed trees, or large wood that create downstream wakes may be important features that provide suitable conditions for the settling of juvenile freshwater mussels onto the river bed. Furthermore, low flows have been increasing since the year 2000 which may be exacerbating the decline in freshwater mussel populations.