The analysis of 1999 discharge data provides information on annual discharge characteristics and the effects of weather systems on discharges for the Broad, Harney, and Shark Rivers. As a part of the U.S. Geological Survey South Florida Placed Based Studies Program, these three estuarine river sites were selected using the criterion that a large amount of the water that flows through Shark River Slough, sometimes referred to as the "Heart of the Everglades," must pass by these sites. This study's data will be used, in conjunction with data from other ongoing studies, to determine the effects changes in water deliveries to Everglades National Park (ENP) have on the southwest estuaries and Florida Bay ecosystems. Each station was equipped with a vertically oriented acoustic-velocity sensor, water-level pressure transducer, bottom water-temperature thermistor, and specific conductance four-electrode sensor.

Discharges from the Broad, Harney, and Shark Rivers are influenced by semi-diurnal tides, wind events, and fresh-water inflow. All three rivers are well mixed, with a difference inspecific conductance from top to bottom usually no greater than 500 microsiemens per centimeter during flood and ebb tides. Discharge is one-dimensional except for brief (less than 20 minutes) periods during slack water (between flood and ebb tide) when flow is vertically bi-directional (moving upstream and downstream). The flood discharges (water moving upstream, denoted as negative values) are usually of greater magnitude and shorter duration than the ebb discharges (water moving downstream, denoted as positive values).

Instantaneous and residual daily discharges for the three stations were calculated for the 1999 calendar year. During 1999, the Broad River instantaneous discharges ranged from -2,400 to +3,500 cubic feet per second (ft³/s), while the Harney and Shark River instantaneous discharges ranged from -15,600 to +12,900 ft³/s and -10,100 to +10,500 ft³/s, respectively. The instantaneous discharge values were processed using a ninth-order Butterworth low-pass filter to remove semidiurnal tidal frequencies that eliminates bias associated with lunar cycles when computing daily, weekly, monthly, or yearly mean or median residual (filtered) discharge values. The residual daily discharges for the Broad, Harney, and Shark River stations ranged from -900 to +2,500 ft³/s, -3600 to +5,700 ft³/s, and -2300 to +4,400 ft³/s, respectively. The Broad River station is the furthest upstream from the Gulf of Mexico (9.3 river miles) and exhibits lesser magnitudes of instantaneous and residual discharges than the other two stations and longer duration positive discharges than the Harney (4.4 river miles upstream) or Shark (6.2 river miles upstream) River stations.

Mean annual residual daily discharges were computed for the Broad and Shark River stations and estimated for the Harney River station. Discharge data were missing for the Harney River from April 4 to June 11, 1999, due to erroneous index-velocity data. This period coincided with prolonged minimum residual discharges recorded at the Broad and Shark River stations. The mean annual residual discharge for the Broad and Shark River stations, using the complete 1999 record were computed as +400 and +440 ft³/s respectively. Excluding the period of missing discharge data for the Harney River station, the mean annual residual discharges for the Broad, Harney, and Shark River stations were +520 ft³/s, +580 ft³/s, and +550 ft³/s, respectively. Applying the same difference of approximately 100 ft³/s between mean annual residual discharges for the Broad and Shark River stations, the Harney River station mean annual residual discharge for 1999 was estimated to be approximately +470 ft³/s. The mean annual residual discharges reflect the net downstream flows with minimal errors associated with water storage.

Wind events such as cold fronts, tropical storms, and hurricanes can amplify, attenuate, or completely overwhelm the tidal forces that normally dominate flow patterns in the estuaries along the southwest coast of the ENP. Four strong cold fronts occurred between January and March 1999 that significantly affected short-term discharges (less than a few days) for the Broad, Harney, and Shark Rivers. The lowest water levels for the Broad, Harney, and Shark Rivers occurred during the passage of strong cold fronts in February and March 1999, when mean water levels were the late summer and early fall. The most significant effects on maximum water level and discharge occurred during the passages of Tropical Storm Harvey and Hurricane Irene in September and October 1999. The two storms had different effects on the water levels and discharges during their movement towards and away from the southwest coast.

Tropical Storm Harvey approached the Broad, Harney, and Shark Rivers from the northwest and moved to the east with maximum sustained winds of 60 miles per hour (mph). The winds associated with Harvey forced water into the mangrove forests of the southwest coast to water levels of approximately 1.81 feet above mean water levels at the Broad River station, 3.30 feet above mean water level at the Harney River station, and 2.96 feet above mean water level at the Shark River station. Some pulsations in water level and discharge not attributable to semi-diurnal tidal forcing preceded the storm by 2 to 3 days. The center storm surge caused a prolonged flood (upstream) flow that lasted almost 24 hours; then as the winds shifted and abated, the stored water flowed back out to the Gulf of Mexico for approximately 24 hours with no tidal flow reversal. The maximum positive and negative instantaneous and residual discharges for the Harney and Shark River stations were recorded on September 21, 1999, as Tropical Storm Harvey made landfall. The Broad River discharges exhibited similar patterns but to a lesser magnitude than the Harney and Shark River stations due to the location of the station and the storm track.

Hurricane Irene caused a different response at the three river stations because of the storm path and wind strength. Hurricane Irene approached from the southwest and moved to the northeast on October 15, 1999, with maximum sustained winds of 85 mph. The winds associated with Irene forced water out of the mangrove forests and the return seiche was of lesser magnitude than during Tropical Storm Harvey. Water levels during Irene decreased and caused a rapid increase in ebb flow (toward the Gulf of Mexico) that lasted approximately 24 hours with no flow reversals during the 24-hour period. The Broad River instantaneous and residual daily discharges reached maximum values of +3,500 and +2,500 ft³/s respectively during the passage of Hurricane Irene.

(This abstract was taken from the Greater Everglades Ecosystem Restoration (GEER) Open File Report (PDF, 8.7 MB))

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