Linking Shifts in Historic Estuarine Vegetation to Salinity Changes Using a GIS
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There are anecdotal reports that upstream water withdrawals over the past 50 years have altered the salinity structure of coastal Georgia estuaries. Since few consistent salinity records exist, it may be possible to use shifts in vegetation to document salinity change. The purpose of this study was to use aerial photographs and GIS analysis to determine if the location of the brackish water interface in two Georgia estuaries has changed. Current vegetation maps of the Satilla and Altamaha estuaries were constructed from 1993 USGS DOQQs. Vegetation was outlined and classified as Juncus roemerianus, brackish marsh, fresh marsh, salt marsh, or other. Historic vegetation maps were similarly constructed from 1:77000-scale color infrared photographs taken in 1974 and 1:24000-scale black and white photographs taken in 1953. Change maps between all years were constructed for each river. In the Altamaha River, 6,786 hectares of marsh area were mapped, of which 77% did not change between 1953 and 1993; Of the 10,205 hectares of marsh area mapped in the Satilla, 87% did not change between 1953 and 1993. Shifts in Juncus constituted the primary vegetation change in both estuaries (95% in the Satilla and 87% in the Altamaha). However, these changes in Juncus do not necessarily reflect changes in estuarine salinity, indicating a need for further investigation of Juncus interactions in these systems.