A study of discharge and nutrient fluxes was conducted at five tidally affected rivers that receive freshwater from the Shark River Slough of Everglades National Park, Florida. The study was funded through the U.S. Geological Survey Placed Based Studies Initiative, and partially funded for the final year by the U.S. Department of the Interior Critical Ecosystem Studies Initiative in cooperation with Everglades National Park (ENP). Data collection began at three stations between November 1996 and January 1997, and two additional stations were added in April 1999.

This study quantified residual discharge and residual nutrient flux from five estuarine rivers along the southwest coast of ENP. This is the first phase of a long-term effort to monitor estuarine river discharge to the southwest coast of Florida and evaluate estuarine discharge from ENP and Big Cypress National Preserve as rehabilitation of the Everglades System progresses.

River discharges were estimated using acoustic velocity systems and linear regression models. Instantaneous discharges for each station varied between -2,600 to +2,400 ft³/s for the Lostmans Creek station; -2,400 to +3,500 ft³/s for the Broad River station; -12,0000 to +10,000 ft³/s for the Harney River station; -10,000 to +8,000 ft³/s for the Shark River station; and -1,100 to +1,100 ft³/s for the North River station. Maximum instantaneous discharge errors were approximately 213, 65, 326, 453, and 193 ft³/s for Lostmans Creek, Broad River, Harney River, Shark River, and North River, respectively. The maximum discharge errors as a percentage of maximum instantaneous discharge are 9, 2, 2, 4, and 18 percent for Lostmans Creek, Broad River, Harney River, Shark River, and North River, respectively. Instantaneous discharge data were filtered using a 9th order Butterworth low-pass filter to remove predominant tidal frequencies, and the filtered data were used to compute daily mean and monthly mean residual discharges. The similarity in residual discharge between Lostmans Creek, Broad, Harney, and Shark Rivers indicates that the Shark River Slough outflow region extends at least as far north as Lostmans Creek, and possibly as far south as the North River. Regression analysis of monthly mean residual discharges indicate that the Lostmans Creek, Broad River, Harney River, Shark River, and North River systems discharge approximately two to three times the amount of water flowing through the Tamiami canal S-12 structures along U.S. Highway 41. Monthly mean residual discharges at the five stations along the southwest coast are at a maximum approximately 1 month before the S-12 structures reach their maximum discharges. Most of the total daily mean residual discharge from the five rivers is evenly distributed between Lostmans Creek, Broad, Harney, and Shark Rivers with each river contributing between 20 to 27 percent of the residual discharge. In contrast, the North River typically discharged only 4 percent of the total residual discharge during the same period.

The concentrations of total nitrogen and total phosphorus were similar in magnitude and range at all five stations; therefore, nitrogen and phosphorus flux distributions resemble the residual discharge distributions. Total nitrogen and total phosphorus residual fluxes from the five rivers were estimated using linear regression models of total nitrogen and total phosphorus flux to instantaneous discharge. Instantaneous nutrient flux data also were filtered using a 9th order Butterworth low-pass filter to remove predominant tidal frequencies, and the filtered data were used to compute residual nutrient fluxes. Total nitrogen residual flux was 1,280 short tons, and the total phosphorus residual flux was 17 short tons for a high flow period between August 1999 and January 2000. The distribution of residual fluxes was similar to the distribution of residual discharges. Lostmans Creek, and Broad, Harney, and Shark Rivers contributed approximately equal amounts of total nitrogen (20 to 27 percent of the total), whereas the North River contributed 4 percent of the total nitrogen. Lostmans Creek, and Broad, Harney, and Shark River stations discharged between 23 to 27 percent of the five station total phosphorus residual flux, whereas the North River discharged 3 percent of the five station total phosphorus residual flux.