The U.S. Geological Survey (USGS) established three monitoring sites south of Lake Okeechobee in an effort to accurately gage flows in canals entering and exiting tribal lands: the Big Cypress National Preserve, and Water Conservation Area 3A (WCA-3A) in southern Florida. These flows are being monitored as part of a multiagency effort to calculate nutrient loads in the canals that cross or border tribal lands. Two of the gaging sites, L-28U and L-28IN, are located on the southern border of the Seminole Indian Tribe of Florida lands along the L-28 canal and the L-28 Interceptor canal, respectively, west of WCA-3A in Hendry County. The third gaging site, L-28IS, is located along the L-28 Interceptor canal where flows enter the western lands of the Miccosukee Indian Tribe from the Big Cypress National Preserve in Collier County.

Acoustic instrumentation, in lieu of standard methods for field data collection and flow computations, is used to gage flows in the canals. With the acoustic velocity meter and the Acoustic Doppler Current Profiler, it is possible to more accurately gage flows in this type of environment because of their capability to quickly measure low or rapidly changing water velocities. Construction, instrumentation, and calibration of the flow gaging sites were completed by the USGS. The South Florida Water Management District (SFWMD) installed flow-weighted samplers at the gaging sites for nutrient analysis in conjunction with the streamflow monitoring; the flow-weighted samplers are serviced by the Seminole and Miccosukee Indian Tribes, respectively. Real-time telemetry programming assistance and phosphorus and nitrogen load calculations are provided by the SFWMD.

The L-28U site along the L-28 canal is used to monitor freshwater flows to and from the lands of the Seminole and Miccosukee Indian Tribes and to provide nutrient data as a source of information for projects by various Federal and State agencies. Total flow was recently determined by the USGS to be 89,000 acre-feet per year, representing twice the inflow amount determined by the SFWMD at their upstream USSO site located on the northwestern border of the Seminole Indian Tribal lands. From September 1997 to April 1998, the flow-weighted mean concentration of total phosphorus was 120 parts per billion at the L-28U site. The total nutrient load was 7,037 kilograms at this site compared to 1,528 kilograms at the upstream USSO site for the same 8-month period. Increases in discharge and nutrient loads at the L-28U site suggest that water, with a total phosphorus content that probably is higher than that from the USSO site, enters the L-28 canal downstream of the USSO site.

The L-28IN site along the L-28 Interceptor canal is used to monitor flows from the lands of the Seminole Indian Tribe to the Big Cypress National Preserve and to provide nutrient data for waterresources planning and management. From September 1997 to April 1998, two sampling methods (autosampler and grab samples) were used by the Seminole Indian Tribe to determine the flowweighted mean concentration of total phosphorus. The flow-weighted mean concentration of total phosphorus determined with the autosampler was 101 parts per billion, and the mean concentration determined from the grab samples was 65 parts per billion. Theoretically, there should be no significant difference between the results as determined by the two sampling methods. Further investigation is in progress to accurately assess quality assurance procedures for weighting of flows with the autosampler.

The L-28IS site along the L-28 Interceptor canal is used to monitor flows from the lands of the Seminole Indian Tribe and the Big Cypress National Preserve to the lands of the Miccosukee Indian Tribe. This site also will be used to provide nutrient data for water managers, and has been instrumental in bracketing and quality assuring the flow calibration conditions for the upstream L-28IN site. Total flow distribution was recently determined by the USGS to be 73,000 acre-feet for the period from September 1997 to April 1998. Total phosphorus results are unavailable at the present time; however, analysis of nutrient loads and subsequent flow-weighted calculations by the SFWMD will be included in the upcoming Fourth Semiannual Progress Report submitted by the SFWMD Headquarters in West Palm Beach.

The implementation of strategically placed streamflow and water-quality gaging sites in the interior of southern Florida provides information for water managers useful to determine future surface-water flow requirements in the interior canal system. Subsequent studies based on accurate flow determinations from these sites will be used for computation of nutrient loads in the interior canal system. Providing continuous flow data at selected critical points for interior basins will compliment that from the eastern flow canal discharge network and allow for surface-water releases that are more accurately timed to deliver water when and where it is needed.

(This abstract was taken from the Proceedings of the South Florida Restoration Science Forum Open File Report)