The fine details of overmarsh circulation remain largely unexplored and yet they are typically assumed to control many attributes of salt marsh material cycling, transport, and accretion. We characterized the spatial and temporal variability in overmarsh circulation at a 2 km2 Georgia, USA, salt marsh using field observations, dye tracer, and numerical simulations. The marsh bathymetry was created with a high‐precision Global Positioning System survey that details the geomorphic structure of intertidal creeks and salt marsh platform features greater than about 1 m in width. We assessed flow path dynamics at four spatial scales ranging from 1 m to 1000 m. Results show the development and decay of simultaneous flow divergence and convergence, concentrated flow and large‐scale rotational flow, and strong differences between flood and ebb pathways. This current complexity is set by submergence and emergence of subtle salt marsh platform geomorphic structure, and it highlights the role of topography in system‐wide flow processes.

Intermittent floodplain channels are low-relief conduits etched into the floodplain surface and remain dry much of the year. These channels comprise expansive systems and are important because during low-level inundation they facilitate lateral hydraulic connectivity throughout the floodplain. Nevertheless, few studies have focused on these floodplain channels due to uncertainty in how to identify and characterize these systems in digital elevation models (DEMs). In particular, their automatic extraction from widely available DEMs is challenging due to the characteristically low-relief and low-gradient topography of floodplains. We applied three channel extraction approaches to the Congaree River floodplain DEM and compared the results to a channel reference map created through numerous field excursions over the past 30 years. The methods that we tested are based on flow accumulation area, topographic curvature, and mathematical morphology, or the D8, Laplacian, and bottom-hat transform (BHT), respectively. Of the 198 km of reference channels the BHT, Laplacian, and D8 extracted 83%, 71%, and 23%, respectively, and the BHT consistently had the highest agreement with the reference network at the local (5 m) and regional (10 km) scales. The extraction results also include commission "error", augmenting the reference map with about 100 km of channel length. Overall, the BHT method provided the best results for channel extraction, giving over 298 km in 69 km 2 with a detrended regional relief of 1.9 m. Further, these analyses allow us to shed light on the meaning and use of the term "low-relief landscapes".

The role of intertidal creek networks in overmarsh circulation is not well constrained. In this study, we systematically remove intertidal creeks from a high‐resolution salt marsh digital elevation model and conduct new flow simulations with each iteration. Overall, removal of first‐ and second‐order creeks reduced drainage density by 65%, and this had a negligible effect on overmarsh circulation and tracer distribution. However, upon removal of third‐ and fourth‐order creeks, drainage density was reduced by 80% of the original value, and changes in peak velocity magnitude across various spatial and temporal scales reveal a system‐wide switch from ebb to flood dominance. This response coincides with the interruption of topographically connected creeks that facilitate the direct exchange of water and tracer between the marsh interior and the larger subtidal system. Further, this reduction in hydraulic connectivity gives rise to less expansive tracer dispersal and a systematic decline in tracer residence time. Together, these results reveal that for the Groves Creek marsh, lower‐order intertidal creeks have a minor role in overmarsh circulation, while higher‐order creeks increase the potential for short circuiting of flow; they greatly influence overall flood and ebb dominance, and net tracer dispersal and associated retention time. Results reported here provide new insight on, for example, salt marsh restoration and the requisite digital elevation model bathymetry for robust simulations of overmarsh circulation.

Low-relief and low-gradient landscapes, particularly along some continental margins, are increasingly susceptible to flooding, and this view is especially relevant given the role of modern climate change in compound flooding (IPCC, 2021). Floodplains are a distinct type of low-relief and low-gradient landscape with a land cover that can range from dense urban centers to largely uninhabited. Although floodplain floods may be considered natural hazards that affect many people at great cost, it has been shown that floodplains and floodplain processes provide substantial benefits to society (e.g., Jakubínský et al., 2021;Mazzoleni et al., 2021;Rak et al., 2016). Therefore, detailed insight on water circulation over inundated areas and the processes that facilitate inundation and drainage can aid in, for example, flood hazard and mitigation analyses and in maintaining floodplain ecosystems and water quality (e.g.,