It is widely accepted that in coastal wetlands a negative relationship exists between plant species richness (number of species) and salinity. However, the distribution of species richness across estuarine salinity gradients has not been closely examined. I hypothesized that plant species richness in coastal marshes (i.e., wetlands dominated by herbaceous plants) would follow a non-linear pattern with increased distance (salinity) downriver (Chapter 2). To test this hypothesis I conducted detailed marsh vegetation surveys along ≈ 50 km estuarine river gradients of the Nanticoke and Patuxent Rivers, MD/DE. I further hypothesized that the observed patterns of plant species richness on the Nanticoke and Patuxent Rivers could be accurately predicted by a mid-domain effect (MDE) model independent of measured abiotic factors using RangeModel 5.0 (Chapter 3). Lastly, I theorized that Marsh mesocosms subjected to intermediate salinity and inundation would exhibit significantly higher biomass and plant species richness compared to mesocosms subjected to extreme salt/fresh and flooding regimes utilizing a controlled greenhouse experiment (Chapter 4). I found that plant species richness can vary in both a linear (Patuxent River) and non-linear (Nanticoke River) pattern along an estuarine gradient. The MDE model did not explain a high proportion of the observed richness patterns for either river system compared to abiotic factors like porewater salinity. The controlled marsh mesocosm experiment supported the non-linear pattern of plant species richness observed along the Nanticoke River gradient, but did not show a significant difference in plant biomass or richness/diversity between purely fresh and low-salinity marsh mesocosms (α = 0.05). The results of this research suggest that tidal marsh plant richness/diversity patterns do not always conform to a simple linear relationship with increasing salinity and that the MDE is not as important of a mechanism in these communities compared to porewater salinity or flooding frequency. Furthermore tidal low salinity marshes exposed to elevated salinity and flooding frequencies are likely to see a shift in their plant community structure to more salt tolerant plants and less rich/diverse communities assuming they can accrete at a rate equal to or exceeding the present rates of sea-level rise in the Chesapeake Bay.