Geomorphic Comparison of Three Globally Significant Wetland Landscapes

Salt marshes are dynamic coastal environments that play a critical role in sediment transport, nutrient cycling, and carbon sequestration. However, the geomorphic factors that influence water flow and material exchange in and between marshes of different size, shape and type remain poorly understood. In this study, we compare the morphology of three distinct marsh landscapes within the ACE Basin, South Carolina: a natural salt marsh (Ethan’s Island), a reclaimed agricultural marsh (Alligator Marsh), and a natural marsh at the upland forest interface (Hannah’s Marsh). Using high-resolution digital elevation models and estimates for drainage density and drainage efficiency, we quantify the similarities and differences in morphology between the different marsh types, and discuss potential implications of our findings in regard to water flow and material exchange. We found that drainage density and drainage efficiency are not always positively correlated and, importantly, the agricultural marsh displays the highest drainage efficiency, despite a drainage density that is comparable to the natural marshes. This finding reveals that the unique linear, interconnected structure of creek networks in the agricultural marsh yields higher efficiency. Further, we found that the natural marsh with the most complex, meandering and fragmented creek networks displayed the lowest drainage efficiency, despite having the highest drainage density. Together, these findings suggest that both drainage density and drainage efficiency should be considered separately, and that drainage efficiency is largely influenced by the structure and spatial arrangement of creek networks with a marsh. Given the relatively higher drainage efficiency of the ag-marsh landscapes, we speculate that such landscapes may enhance the flow of water and sediment between inland areas and the coastal ocean, a process that can help marshes migrate landward as sea levels rise.