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The Great Marsh in northern Massachusetts and southern New Hampshire is the largest salt marsh in New England and includes the marsh surrounding Plum Island Sound, one of the most studied coastal wetlands in the United States. A primary goal in marsh research today is predicting their resilience in a regime of sea-level rise, which has become increasingly based on modeling studies with sparse or site-specific field data input. Here, we present the results of a vertical accretion study using 210Pb and 137Cs geochronology developed from 50-cm long auger cores taken at 15 locations throughout the PIS marsh. Two significant outliers were omitted from the final analyses due to their unusual depositional settings. The 137Cs spike produced the shortest-term record and lowest average accretion rate (2.5 mm/yr). Of the various 210Pb accretion models (Constant Flux-Constant Sedimentation: CF-CS, Constant Rate of Supply: CRS, Constant Initial Concentration: CIC), we chose the CF-CS model accretion rate for comparison to other environmental parameters and future marsh projections due to the robust R2 linear regression values and its wide use. The 13 stations exhibited an average accretion rate of 3.13 mm/yr, which is 0.6 mm/yr greater than the average rate reported elsewhere in New England. Individual accretion rates show no correlation with geography, elevation, bulk density, loss on ignition, distance to tidal channels, or peat thickness, indicating the complexity of the interacting factors that control marsh growth. Using moderate sea-level rise predictions, we show that large portions of the high marsh are likely to be converted to low marsh by 2050, and that the entire marsh high platform will become low marsh prior to 2070. This is the first time that wide spread field data are used to project a major transformation of the Plum Island Sound platform marsh that undoubtedly will lead to dramatic changes to marsh drainage, backbarrier hypsometry, tidal current dominance, and redistribution of sediment reservoirs, that will include the fronting barrier islands. [ABSTRACT FROM AUTHOR]