EFFECTS OF RIVER DISCHARGE AND HIGH-TIDE STAGE ON SALINITY INTRUSION IN THE WEEKI WACHEE, CRYSTAL, AND WITHLACOOCHEE RIVER ESTUARIES, SOUTHWEST FLORIDA

The Weeki Wachee, Crystal, and Withlacoochee Rivers are coastal streams flowing into the Gulf of Mexico that may be affected by either future surface-water or ground-water withdrawals. Reduction of river discharge may affect the upstream extent of saltwater intrusion in the three rivers; however, under certain reduced low-flow discharges, the estimated change in upstream extent of saltwater intrusion is on the order of several tenths of a mile and frequently is within the range of predicted error. Data on river discharge, high-tide stage, and salinity are used to characterize salinity distributions in the Weeki Wachee, Crystal, and Withlacoochee River Estuaries. Salinity of the rivers increases downstream. The difference between surface and bottom salinities varies from river to river. Vertical differences in salinity are smallest in the Weeki Wachee River and largest in the Withlacoochee River Estuary. Multiple linear-regression analysis was used to relate the maximum upstream extent of the vertically averaged 5and 0.5-part-per-thousand salinities in the Weeki Wachee and Withlachoochee Rivers and the vertically averaged 5and 2-parts-per-thousand salinities in Crystal River to daily discharge of each river and high-tide stage of the Gulf of Mexico. For the Weeki Wachee River, two linear equations were developed for different ranges of discharge for each salinity position. Streamflow was the most significant variable in each equation. For the Crystal River, one linear equation was developed for each salinity location. High-tide stage was the most significant variable in each equation. For the Withlacoochee River, linear equations were developed for the surface, bottom, and vertically averaged salinity locations. Discharge was the most significant variable in locating the bottom salinity position, and high-tide stage was the most significant variable for locating the surface salinity position. The mean location of both salinities responded slightly more to tide stage than to discharge. Pumping 40 million gallons per day (61.9 cubic feet per second) from a well field near the springs would result in a 43-cubic-foot-per-second reduction of discharge in the Weeki Wachee River. This reduction is estimated to cause an upstream movement of the vertically averaged 5and 0.5-part-per-thousand salinities of about 0.3 and 0.2 mile, respectively, at the 25-percent flowduration interval (199 cubic feet per second). The frequency of a salinity intrusion greater than 0.5 part per thousand at mile 1.5 is estimated to increase from 6 to 65 percent. The well-field pumpage would result in a 61-cubic-foot-persecond reduction in discharge in the Crystal River. The reduction inflow would cause an insignificant upstream movement of the vertically averaged 5and 2-parts-per-thousand salinities at the 90-day, 20-year low flow. Withdrawal of 208 cubic feet per second (50 percent of the 90-day, 2-year low flow) from the Withlacoochee River is estimated to cause an upstream movement of both the vertically averaged 5and 0.5-part-per-thousand salinities of about 0.2 mile. It would also increase the frequency of daily salinity intrusion (0.5-part-per-thousand salinity) at mile 3.0 from 33 to 85 percent. For the 7-day, 2-year low flow (670 cubic feet per second), 50-percent reduction of flow would cause an upstream movement of the vertically averaged 5and 0.5-part-per-thousand salinities of about 0.4 and 0.3 mile, respectively.