The three-dimensional structure of a salt marsh island

Studies have shown marsh topography exerts considerable control on the timing and duration of platform inundation. However, topographic structure of the intertidal salt marsh remains poorly quantified. In this study, I used a survey grade Digital Elevation Model (DEM) to accurately quantify the spatial variability of topographic structures including the 1D, 2D, and 3D geometry of 23 tidal creeks across a 0.5 km2 island within the intertidal zone of North Inlet South Carolina, USA. A histogram of elevations shows 92% of the island is above mean sea level and only completely inundated by water levels greater than Mean High High Water (0.66 m NAVD88). Topography across the study site varies within a 2.1 m range and has a distribution of 29% levee, 63% vegetated platform, 6% creek, and 2% surface depression. Intertidal creek properties are highly variable as well, but significant relationships exist, for example creek mouth depth : total creek length, and creek mouth area : creek area (p<0.001). Observed values between the relationships of channel width : distance upstream, sinuosity : relative age, and creek mouth depth : tidal datum, differ from values predicted by salt marsh morphology models. Differences in creek mouth depth, channel slope, and hypsometry are attributed to newly formed creeks, which appear to be extensions of preexisting tidal flat creeks. The creek data presented in this study is the most accurate and expansive to date. Furthermore, the high resolution DEM shows an accurate distribution of topographic structure across an intertidal marsh that was previously unattainable. Results gleaned from this study are significant enough to bring into question the validity of conclusions drawn from previous studies within North Inlet that investigated sediment accretion, transport and redistribution, as well as the distribution of organic matter to include vegetation density.