Use of a date-specific method to examine variability in the flushing times of Georgia estuaries

The fraction of freshwater method (Dyer, 1973) was used to examine the general characteristics of flushing time in five riverine estuaries in Georgia, U.S.A. Median flushing times over a 30-year period (1968-1997) were as follows: Savannah (5.6 days), Ogeechee (20.7 days), Altamaha (5.8 days), Satilla (66.8 days), and St. Marys (71.6 days), although there was considerable inter- and intra-annual variability in these estimates. For estimation of intra-annual flushing times, a dare-specific method is used in which it is assumed that the appropriate time period over which to average discharge is equivalent to the flushing time. Although still technically a steady-state calculation, this approach improves flushing time estimates. When applied to a 9-year data set for four of these estuaries (all but the Savannah), the date-specific method yielded different flushing times than those calculated using a fixed discharge period, especially during periods of low or abruptly changing discharge. Flushing time is the freshwater volume divided by discharge; therefore, the relationship between flushing time and discharge is a power function: at fast discharge rates flushing time is relatively stable, but as discharge slows flushing time increases quickly. As a consequence of this relationship, (1) decreases in discharge have proportionately more impact on flushing times than do increases, and (2) flushing times in estuaries with slower discharge are more sensitive to changes in river flow. (C) 1999 Academic Press.