QUANTITATIVE GEOMORPHOLOGY OF A NEW ENGLAND ESTUARINE SALT MARSH SYSTEM USING MULTISPECTRAL IMAGERY, GIS AND LIDAR

Salt marsh processes are often described and quantified based on localized observations within a complex and highly heterogeneous system. In New England, backbarrier and estuarine intertidal zones are commonly dominated by the high marsh ecotone which develops above mean high water. Salt marsh vegetative diversity is greatest in these “meadow marshes,” which also exhibit significant microtopographic, sedimentary, and biochemical variance. The largest estuary in New England is the Great Marsh, Massachusetts, USA, a 25 km-long wetland-dominated mesotidal backbarrier system. Using 20cm AIMS-1 multispectral imagery, flown at spring high and low tides, together with a LIDAR-derived DEM of the study area yields new insights into relationships between marsh features, their metrics, elevations, and tidal datums. Even across generally low-relief systems such as tidal wetlands, this study finds LIDAR to be capable of resolving microtopographic variation which may be a critical component of net surface roughness across the vegetated marsh platform. High-resolution tasked imagery also improves significantly on remote feature recognition and classification, enhancing the extraction of system-wide parameters useful for coastal management, including elevation of vegetative zones, distribution and qualities of standing water on the marsh surface, and tidal network characteristics. For example, results quantify the density of anthropogenic ditching required to prevent pond and panne formation as ~8-10% of surface area in the high marsh; this may be taken to empirically suggest an optimum efficiency of tidal flushing for Spartina patens, which dominates the marsh platform in the study area. Comparison between natural-creek and ditched portions of the marsh at similar elevations within the tidal range supports this premise, which requires continuing hydrodynamic and ecological investigation. Further, quantifying the elevation of the ‘headwaters’ of natural tidal creeks across the entire system may provide a sensitive benchmark for evaluating hydrodynamic response to sea-level rise across the linked estuarine-coastal system.