Response times of salinity in relation to changes in freshwater inflows in the Lower Hillsborough River, Florida

The Lower Hillsborough River, Florida is a short (16 ion) riverine estuary which has a dam located at its upstream end. Salinity below the dam is influenced by freshwater that hows over or through the structure. Depending on location in the estuary, the response of salinity to changes in upstream freshwater inflows is normally not instantaneous, but lags behind the freshwater release. An analytical approach and a laterally averaged two-dimensional hydrodynamic model were used to examine the response time of salinity in the Lower Hillsborough River to changes in freshwater inflows from the upstream reservoir. A series of case studies were conducted using the model to determine how salinity in the river within one kilometer below the dam would respond to changes in freshwater inflows. The model results suggest that the time lag of salinity in the river depends on whether the upstream freshwater inflows are increasing or decreasing, as wed as their magnitude. While the time lag for salinity is about six to eight days for decreasing inflows, it is much shorter for increasing inflows depending on the magnitude of the now release.