The damming of James Bay as proposed by the grand Canal Scheme will influence the hydrology of the coastal marshes. The salinity regime of two James Bay marshes were compared to understand the processes that govern salinity in the zone near mean high water (mhw) and to predict changes that may result from this proposed impoundment. Data suggest that sites along southern James Bay would be minimally affected because of the current low level of salinity. The northern marshes would, however, experience significant changes, as their primary source of salt is from saline tidewater. A simulation of the vertical salt and water fluxes at a northern location predicts a rapid reduction in the chloride concentration at mhw, which should approach the present values for southern James Bay in about 10 years. Above mhw, the salt will be leached quickly, so that within four years the surface chloride concentration will be reduced to the level currently found in southern James Bay.