Measurements of soil salinity, soil moisture, and specific yield, along with depth to the water table, explain the occurrence of a band of hypersaline sediments, ubiquitous along James Bay coastal marshes, that profoundly affects vegetation development. The salinity of the hypersaline zone ranges from 8 g/kg Cl in a southern James Bay marsh, to 28 g/kg in the north, nearly an order of magnitude ... [Show full abstract] more than local sediments outside this zone. This common, yet previously unexplained, feature restricts the vegetation development to salt-tolerant species such as Salicornia maritima, Puccinellia phryganodes, and Triglochlin maritimum. The hypersaline zone is rarely inundated by tides, but frequent inundation of the seaward zone maintains a high water table there, with soil salinity, hence vegetation patterns that reflect the ambient tidewater salinity. At sites inland of the hypersaline band the development of an organic layer with a high moisture content supplied by meteoric water and high specific yield maintains relatively fresh water near the surface, imposing minimal salinity restrictions to vegetation. Intermediate to these zones of high water table is the hypersaline band, which has no organic layer and whose surface is not wetted by tides. Its depressed water table causes flow convergence, and the only water sink is through evaporation, which concentrates the salt imported in groundwater. Key words: James Bay, marsh, salinity, hydrology, vegetation.