Spatial variability in hydrologic connectivity between a coastal river and its floodplain

Significant in the transport and dispersal of energy, matter, and biota within riverine environments, hydrologic connectivity has become fundamental in understanding river-floodplain process and concepts associated with environmental flows. Hydrologic connectivity between a river and its floodplain is dependent on floodplain geomorphology, channel hydraulics, land cover, and groundwater dynamics. In this study, we examine the floodplain morphology of the Mission River on the Coastal Bend of Texas. Utilizing a LiDAR-derived digital elevation model, we develop a relative digital elevation model (RDEM), with respect to river-stage, for a reach of the river-floodplain system. To assess the dominant flow directions, and thus, characterize the connectivity within the floodplain, we determine hydrological facets—landscape patches with their own respective outlet and high internal surface water connectivity. Guided by historical streamflow records, we systematically threshold the RDEM to determine the spatial characteristics of floodplain inundation under various river-stages. The increasing aerial extent of floodplain inundation from increases in river-stage results in distinct patterns of river channel-floodplain connectivity. We analyze the spatial arrangement of facets with regard to inundation in order to quantify floodplain connectivity and complexity at various river-stages using graph theory and landscape metrics. Our results indicate that floodplain connectivity and complexity change nonlinearly with increases in river-stage; with the greatest increases in connectivity and complexity occurring well below bankfull discharge. Floodplains provide valuable ecosystem services and this study helps improve our understanding of process-form relationships in river-floodplain systems' that informs environmental policy for effective management of these services.