Correlation of Freshwater Discharge and Subtidal Salinity in Apalachicola River

Modification of river flow from engineering activities often affects salinity and the mixing of fresh and saltwater in tidal rivers. In this study, an empirical relation between subtidal salinity and gradually varying freshwater inflow at a given station in the Apalachicola River, Florida, was developed and tested with field observations. A time series of subtidal salinity was obtained by low-pass filtering of the instantaneous salinity to eliminate the tidal signal. Based on the one-dimensional balance theory between advection and diffusion of salt, the time series of subtidal salinity was directly correlated to the gradually varying freshwater input by an exponential equation. With field observations obtained from the Apalachicola River, the empirical constants were obtained by regression. The correlation between observations and the results from the empirical equation were very good. This simple empirical relation may be used by water resource engineers and managers in preliminary studies to quickly estimate the potential impact of water resource planning on subtidal salinity at a given location in a tidal river.