At each site, three transects were measured out from the marsh edge to the low tide line (marked as 1 ,2 and 3 in the data file ) Each transect was divided into 5 (sometimes 6 depending on the length of the transect) points (marked as A-E, sometimes F in the data file) and at each point the elevation of the point was calculated. The elevation for each sampling point was extracted from the digital elevation model (DEM) of Plum Island Sound, 2 m spatial resolution, developed using LIDAR data (Zhao, Chen et al. 2010). A one square foot quadrat was placed on the sediment and any clams residing in that section of sediment were counted to determine the density of clams living at that specific elevation in the sediment (density data is reported as number of clams per square meter). Each clam found was measured for length and weight. Midtransect sediment samples were taken to determin e the sediment grain size at each site/transect. Sediment samples were analyzed in the lab to determine the percentage of pebbles, (phi size 0 to -6) sand (phi size 0 to 5), clay (phi size 6-9) and silt (phi size >9) in each .A detailed procedure for determining sediment grain size and the theory behind the calculations can be found at the following: http://pubs.usgs.gov/of/2000/of00-358/text/chapter1.htm http://www.lib.utexas.edu/geo/folkready/entirefolkpdf.pdf Briefly, a homogenized subset of sediment was taken from each bagged sample and dried in a drying oven. The driedsediment was then shaken through a series of mesh sieves and the sediment remaining in each sieve was weighed. Anything larger than 2000um was considered gravel/pebbles, and anything smaller than 2000umand larger than 63um was considered part of the sand fraction. Anything that ended up in the smallest mesh size sieve (<63um, the clay + silt fraction) was placed into a 1000ml graduated cylinder with some dispersant (sodium hexametaphosphate, 50g/L) The cylinder was shaken through inversion for 1 minute and then allowed to settle over a 24hr time period. During that time frame the cylinder was subsampled at set intervals using a volumetricpipette at a specific measured depth in the cylinder. This liquid was put into a pre-tared beaker and the beakers were dried in a drying oven and then weighed . Calculations outlined in http://www.lib.utexas.edu/geo/folkready/entirefolkpdf.pdf and http://pubs.usgs.gov/of/2000/of00-358/text/chapter1.htm were used to determine the percentage of pebbles, sand, clay and silt in each sample.