Monitoring station inspection [larger image]

Changes in water-management practices have been made to accommodate a large and rapidly growing urban population along the Atlantic Coast and to meet the demand for intensive agricultural activities. These changes have resulted in a highly managed hydrologic system consisting of numerous canals, levees, control structures, and pumping stations that have altered the hydrology of the Everglades and Florida Bay ecosystems. Over the past decade, Florida Bay has experienced sea-grass die-off and algal blooms, which are indicators of ecological change attributed primarily to the increase in salinity and nutrient content of bay waters. Because plans are to restore sheetflow in the Everglades wetlands to its natural state, water managers anticipate a change in the magnitude and timing of freshwater exiting the mainland through the creeks that cut through the embankment or as sheetflow into Florida Bay.

Figure 1. Location of monitoring stations in northeastern Florida Bay. [larger image]

Flow through the mangrove zone into northeastern Florida Bay is controlled by water levels in the Everglades wetlands, regional wind patterns, and to a minor extent, tidal fluctuations. Restoration of the Florida Bay ecosystem requires an understanding of the linkage between the salinity, quality of the bay environment, and the amount of freshwater flowing into the bay.

In 1995, the U.S. Geological Survey (USGS) initiated the South Florida Ecosystem Program (now known as the South Florida Place-Based Program). This program was established as a collaborative effort by Federal agencies working with State and local agencies to resolve land-use demands and water- supply issues in southern Florida. As part of this program, the USGS is presently conducting a study to measure flows into Florida Bay from the mainland through the major creeks along the northeastern coast. The results of this study will provide scientists with essential information from along the mangrove zone where data were not previously available.

Freshwater flow into north- eastern Florida Bay is mostly confined to several creeks, except during extreme high-water conditions when sheetflow could occur in low-lying mangrove areas between the creeks. Five monitoring stations in the mangrove zone along the northeastern coast of Florida Bay (fig. 1) were selected to determine the magnitude and distribution of flows into the bay. The West Highway Creek, Trout Creek, Mud Creek, Taylor River, and McCormick Creek stations are located along a mangrove zone, spanning about 20 miles across the northeastern edge of Florida Bay. Stage, water velocity, temperature, and salinity data were collected from these stations. Figure 2 shows a typical USGS monitoring station (Taylor River) in Florida Bay.

Time-series records of acoustic velocity were collected and used with discharge measurements to establish an acoustic-velocity to mean-velocity relation for the creeks. The technique used to describe the velocity relation is a least squares regression model in the single or multiple form, depending on the number of identified significant variables. The model uses the following equation:

where is mean channel water velocity, V_l is acoustic line velocity, X₁ and X₂ represent regression coefficients, H is stage, and C is a constant.

The acoustic line velocity is the predominant variable, and in most cases, the only variable needed for the description of the velocity relation. Occasionally, stage plays a significant role in describing the relation and needs to be included in the above equation. A typical acoustic-to-mean velocity relation for one of the monitoring stations (Trout Creek) is shown in figure 3.

Velocity and area relations developed for each monitoring station were used to calculate 15-minute time-series discharge data for all gaged creeks along the coastline. In other areas, most estuarine creeks present flow patterns that include the reversal of flow direction during flood and ebb tides. In the creeks that discharge into Florida Bay, this typical tidal signature is either nonexistent or substantially altered by wind forces. The flow direction for stations monitored in northeastern Florida Bay is mainly dictated by the wet- or dry-season conditions of the Everglades wetlands and the regional wind speed and direction. Commonly used data filters were not used for net flow calculations because of these flow patterns. Monthly mean values of discharge were used to account for storage and to determine net flows for all monitoring stations.

Discharge results for the five stations indicated that the majority of flow into Florida Bay occurs east of the Taylor Slough drainage area. Discharge was least for McCormick Creek (3 percent of total) and greatest for Trout Creek (67 percent of total) during May 1996 through April 1998. The percentages of freshwater contributions for the five monitoring stations are shown in figure 4.

By Eduardo Patino and Clinton Hittle

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