Using Hydrogeomorphic Features to Quantify Structural and Functional Hydrologic Connectivity in a Coastal Plain Headwater Stream

Headwater streams comprise most of the global river length, and hydrologic processes occurring in headwaters affect the chemical, physical, and biological functions of downstream aquatic ecosystems. However, we do not have a clear understanding of the spatial scales that drive hydrologic processes across headwater systems, particularly in Coastal Plain landscapes. We address this gap by characterizing hydrologic connectivity in a small, forested watershed in the Coastal Plain of Alabama, USA. We collected data across three spatial scales: the watershed (0.9 km2), hydrogeomorphic feature (100-500 m), and hillslope (10-100 m) scales. We characterized stream network variability using seasonal surveys combined with water monitoring wells to characterize stream hydrologic state across 2021, paired with an Electrical Resistivity Tomography (ERT) and Time Domain Induced Polarization (TDIP) survey to characterize subsurface structure. Our results suggest that discretizing the river corridor into distinct hydrogeomorphic features provides a framework for understanding the dynamics of hydrologic connectivity within a watershed. Each hydrogeomorphic feature experienced consistent hydrologic states that differ along the network: incised channels gained water, intact riparian zones lost water, and wetland-stream complexes reflected no net water gain or loss from the river corridor. Subsurface structures observed with the ERT/TDIP survey indicate heterogeneous perched flowpaths, with saturation occurring variably throughout both space and time. Altogether, these results suggest that studying watersheds across a hierarchy of scales can provide insight into the dynamics of hydrologic connectivity, and that hydrogeomorphic features can provide a key intermediate scale for the integration of hydrologic processes across the river corridor.