WRIR 99-4268

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Tillis, G.M., 2000, Flow and salinity characteristics of the upper Suwannee River Estuary, Florida: Water-Resources Investigations Report 99-4268, 40 p.


Continuous stage and salinity data were recorded from August 1995 to December 1997 at four gages located in the upper Suwannee River Estuary. Continuous velocity data were recorded at two of the four gages and continuous discharge data were computed for these two gages. Additional salinity data were collected at 15 monitoring sites from November 1992 to October 1997. Wind-speed data collected at Cedar Key, Florida, during the study period were utilized in the regression analysis. Correlations were developed to describe the longitudinal extent of the saltwater/freshwater interface (defined as 0.5 parts per thousand (ppt) salinity) and salinity distribution in the upper Suwannee River Estuary. On East Pass, the median of difference between daily maximum and daily minimum stage ranged from 2.92 feet for a gage at river mile 3.8 to 3.33 feet for a gage at river mile 1.2. Velocities tended to be unidirectional with some instances of bilateral flow. Reversal in flow direction was common and coincided with rising tides. Monthly mean discharges for the Suwannee River near Wilcox, Florida, during the study period typically were lower than the average for the period of record (1931.97). Discharge near Wilcox averaged 4,000 cubic feet per second (ft3/s) lower than the long-term average from June to September 1996. An El Niņo event induced precipitation that was responsible for higher than average monthly mean discharge measured near Wilcox during November and December 1997.

The maximum observed salinity concentrations for the study period ranged from 28.20 ppt at river mile 3.8 to 31.00 ppt at river mile 1.9. Median daily fluctuations of salinity at river miles 3.8 and 1.2 were 0.12 and 11.31 ppt, respectively. The maximum daily upstream extent of the saltwater/freshwater interface was at or upstream from river mile 4.0 for about 50 percent of the study period. The interface was at or upstream from river mile 3.8 and river mile 2.8 40 and 57 percent of the time. The interface was downstream from river mile 1.2 and river mile 1.9 11 and 21 percent of the time, respectively. The median daily maximum salinity for the four gages ranged from 0.22 ppt at river mile 3.8 to 11.50 ppt at river mile 1.2.

Multiple linear-regression models were developed to determine the isohaline location for 0.5, 2, 5, 10, 15, and 20 ppt salinity, and to predict the maximum daily salinity concentrations at gages as a function of stage, river discharge, and wind. The salinity at a location was inversely proportional to the daily mean discharge at the Suwannee River near Wilcox. Under extreme low-flow conditions (3,500 ft3/s), the regression models predicted that the interface would occur at river mile 7.2, upstream from the Gopher River confluence with the Suwannee River. Wind speed did not have a substantial influence on model predictions.

The period of record for the Suwannee River at Wilcox was applied to appropriate regression models to produce a synthetic record of historical salinity distributions. Two withdrawal scenarios, a 10-percent diversion and a 1,000 ft3/s diversion, were evaluated relative to high-, medium-, and low-flow conditions and compared to actual salinity distributions. The 10-percent and 1,000 ft3/s withdrawals scenarios resulted in the isohaline of 0.5 ppt migrating 0.6 and 1.58 miles upstream from the actual isohaline location for a low-flow condition of 4,500 ft3/s, and migrating 0.14 and 0.65 miles upstream from the actual isohaline location for a high-flow conditions of 20,300 ft3/s for Wadley Pass.