Hydroperiod and salinity are the major variables that determine species composition and ecosystem structure along the continuum from intertidal marsh to upland. Hydrodynamics and water sources along this continuum are not adequately documented and quantified in ways that would relate to such ecosystem processes as primary production, decomposition, sediment accretion (both organic and inorganic), and plant community dynamics. By positioning water level recorders and groundwater sampling wells along this continuum, we can begin to identify water sources (precipitation, groundwater, estuary) and the manner in which they are delivered to specific sites in the marsh (atmospheric, subterranean, lateral surface and near-surface flow).