Assessing salinity induced changes to tidal freshwater and oligohaline marsh habitats in the lower Savannah River.

The Savannah River is a large river of the southeastern US coastal plain. It flows from the mountains of western North Carolina eastward to the Atlantic Ocean. Some 40 miles prior to reaching its discharge point, the Savannah River begins assuming a tidal signal, which is amplified, to over 7 feet further down river. This extreme tide range allows the vestiges of the estuarine salinity gradient to be detected over 25 miles up river. In this area, the combination of tidal action and very low salinity levels has allowed the development of tidal freshwater and oligohaline marshes. The dividing line between these two marsh types is defined as the point along the salinity gradient where the average annual salinity is 0.5 ppt. While highly favored by wildlife and an important component of the estuarine food web, these tidal freshwater and oligohaline marshes are exceedingly rare because they are located at the pivot point of an environmental seesaw that balances the rise and fall of the saline ocean tide on one side and the flow of the freshwater Savannah River on the other. The narrow range of conditions conducive to tidal freshwater and oligohaline marshes exists for a only a few miles along the Savannah River and confines these habitats to a narrow band hemmed in by forested swamps upstream and salt marshes downstream. These marsh habitats are largely incorporated into the Savannah National Wildlife Refuge (SNWR). Management of this resource must consider the continued development and deepening of the nearby Savannah Harbor, a major port. There is a concern that deepening of the shipping channel will allow the salinity gradient to extend further upstream and, consequently, adversely impact the tidal freshwater and oligohaline marsh habitats. To address this concern, a comprehensive habitat assessment is ongoing to address the spatial distribution of the tidal freshwater and oligohaline marshes in relation to the salinity gradient. Marsh habitats were mapped using SPOT satellite imagery in combination with aerial imagery and ground field surveys. Image processing techniques were used to classify the satellite imagery into different habitat types. These habitats types were verified by field ground-truthing. Differentially corrected GPS was used to verify locations within the tract less expanses of the marshes. The location of the salinity gradient was determined through hydrodynamic modeling. The salinity model accounted for the fluctuations in the gradient locations resulting from tidal action and river flow. Model results were expressed as the 50th percentile of the 0.5 ppt salinity contour. Additional modeling of a post-deepening condition provided the upstream offset of this 0.5 ppt. contour, bracketing the area of potential impact. Quantitative vegetation sampling across the salinity gradient provided data regarding plant distribution within the gradient. These data were extrapolated to the potential impact zone to predict potential vegetation shifts resulting from salinity changes. Ongoing work is focusing on further defining the linkage between salinity levels in the river channel and the marsh, as well as providing a better estimate of the rate of vegetation change.