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. / Doctor of Philosophy / Human civilization has developed near rivers due to the wide range of benefits provided by rivers. Rivers provide food, water, and energy to more than 2.7 billion people around the world. However, the health of the rivers is degrading rapidly to meet the increasing demand of the growing population. We studied water, sediment, and mussel transport in the three rivers in the US: East Fork White River in Indiana, South River in Virginia, and Dan River in North Carolina. These rivers play an important role in agriculture, water supply, sediment, and nutrient transport of the surrounding environment. Our research work on East Fork White River in Indiana, USA shows that the area directly adjacent to the river is eroding, which is important information for river managers and policymakers. As part of that work, we identified the potential of various sizes of sediment to move over this area at different flows and developed a method to predict the largest sediment size that could be moved in water and hopping along the ground. This method is also applicable to other areas along rivers and the coast. We estimated the sediment deposition rate, deposition volume, and prepared a spatial map of the sediment deposition pattern for the South River floodplain in Virginia. From this map, deposition hot spots could be identified. We estimated that 66% of the sediment deposited adjacent to the South River was located in 32% of the area. This information will be helpful for understanding how sediment and sediment-associated pollutants deposit around rivers. Our work on the Dan River in North Carolina was focused on freshwater mussels. Our results showed that juvenile freshwater mussels could not have settled onto the river bed at the location of our measurements. Historical data reveal that freshwater mussels are declining at an alarming rate in that river, posing a threat to the river environment. We identified that streamflow has been increasing over the last two decades, which could be a potential cause of declining freshwater mussels.

Rivers and their surrounding riparian and subsurface ecosystems, known as river corridors, are important landscape features that provide a myriad of ecological and societal benefits. While the importance of riverine flooding has been widely acknowledged and extensively studied, very little research has been conducted on the interactions between river channels and their adjacent floodplains. The importance of this hydrologic connectivity between rivers and floodplains has been emphasized in recent decades and now ecological engineering techniques such as stream restoration are often utilized to restore connectivity between streams and their riparian ecosystems. Despite its ubiquity in practice, there are still many basic components of river-floodplain connectivity that are not well understood. Furthermore, a lack of cost-effective monitoring techniques makes sustainable management of river corridors quite challenging. Thus, the overall goals of my dissertation were: 1) develop user-friendly river corridor monitoring techniques utilizing cost-effective approaches such as time-lapse digital imagery and satellite remote sensing and 2) identify the effects of anthropogenic activities on river corridor hydrologic and biogeochemical processes that occur at varying spatial and temporal scales during flood events. These goals were addressed through five independent studies that span spatiotemporal scales. The five studies utilized a combination of novel remote sensing, hydrologic/hydraulic modeling, and high frequency spatial sampling techniques to analyze river corridor dynamics. Results highlight that digital imagery and satellite remote sensing can be effective tools for monitoring river corridors in data scare regions. Additionally, impounding streams and river corridors alters floodplain connectivity and biogeochemical processing of reactive solutes such as nitrogen and phosphorus. Findings from this work highlight the important role that spatial and temporal scale plays in river corridor dynamics. Overall, this research provides new analytical techniques and findings that can be used to effectively monitor and manage river corridors. / PHD / Rivers are important landscape features that provide basic societal needs such as drinking water, water for agricultural irrigation, and hydroelectric energy. Engineers have traditionally sought to manage rivers for these purposes while also minimizing flooding. However, flooding actually provides a number of environmental benefits such as increased aquatic biodiversity and removal of excess sediment and pollutants from rivers. This notion of environmentally friendly flooding is a relatively new concept and much is still unknown about how these processes differ at varying scales. Additionally, there is currently a lack of techniques for monitoring such processes primarily due to the cost required for equipment and labor. Therefore, the goals of this dissertation were twofold: 1) develop cost-effective and user-friendly monitoring techniques that can be used to study river flooding dynamics and 2) examine the impacts of river flooding dynamics at three different spatial scales ranging from a small stream to a large watershed. This was accomplished through five separate case studies that examine rivers and watersheds of varying sizes at varying time scales. The studies utilized several emerging technologies that required a combination of field monitoring, computer simulations of flood dynamics, and satellite imagery to gain a better understanding of river flood hydrology and water quality. A key finding was the important role that scale plays in both spatial and temporal domains. Utilizing varying spatial and temporal scales allowed for identification of different processes that occur across a range of river and watershed sizes. Overall, this work can be used to better inform future river management and restoration decisions.

Floodplain inundation often provides water quality benefits by trapping sediment and biogeochemically transforming other pollutants. Hydrologic floodplain connectivity is a measure of water exchanges and interactions between the main channel and the floodplain via surface (inundation) and subsurface (groundwater) connections. Using an automated model combining GIS and numerical analysis software, this study examined floodplain inundation patterns and measured floodplain connectivity for the Mahantango Creek watershed (Pennsylvania, USA). Connectivity was quantified by developing a metric that included inundation area and duration. Long-term hydrographs at each reach in the watershed were developed via QPPQ (Flow-Percentile-Percentile-Flow) methodology using regional regression analysis to calculate the ungauged flow duration curves (FDC). Inundation area (normalized to stream length) was found to increase with drainage area, suggesting larger streams have more area available for biogeochemical activity. Annual connectivity increased with drainage area, suggesting larger streams, having higher connectivity, should be the focus of individual reach restoration projects due to higher potential for water quality benefits. Across the watershed as a whole, however, the total annual connectivity across first order streams was greater than higher order streams, suggesting the collection of small streams in a watershed may have a stronger effect on outlet water quality. Connectivity was consistently higher during the non-growing season, which was attributed to higher flows. Despite higher connectivity during the non-growing season, increased floodplain biological activity may be negated by low temperatures, reducing microbial activity. Correlations between land use and connectivity were also found, emphasizing dynamics between flow, channel morphology, and floodplain inundation. / Master of Science

River-floodplain connectivity is defined as the water mediated transfer of materials and energy between a river or stream and its adjacent floodplain. It is generally accepted that restoring and/or enhancing river-floodplain connectivity can reduce the downstream flux of reactive solutes such as nitrogen (N) and phosphorus (P) and thus improve downstream water quality. However, there is little scientific literature to guide ecological engineering efforts which optimize river-floodplain connectivity for solute retention. Therefore, the aim of my dissertation research was to examine feedbacks between inundation hydrology and floodplain biogeochemistry, with an emphasis on analyzing variation experienced along the river continuum and the cumulative effects of river-floodplain connectivity at the basin scale. This was completed through four independent investigations. Field sites ranged from the Atchafalaya River Basin, the largest river-floodplain system in the continental US, to the floodplain of a recently restored headwater stream in Appalachia. We also developed a method to examine river-floodplain connectivity across large- river networks and applied that methodology to US stream network. Largely, our results highlight the role floodwater residence time distributions play in floodplain biogeochemistry. In headwater streams, residence times restrict redox dependent processes (e.g. denitrification) and downstream flushing of reactive solutes is the dominant process. However, in large-river floodplains, redox dependent processes can become solute limited because of prolonged residence times and hydrologic isolation. In these floodplains, the dominant process is often autochthonous solute accumulation. Further, results from our modeling study suggest large-river floodplains have a greater impact on downstream water quality than floodplains associated with smaller streams, even when considering cumulative effects across the entire river network. / Ph. D.