Historical changes in water-management practices to accommodate a large and rapidly growing urban population along the Atlantic Coast, as well as intensive agricultural activities, have resulted in a highly managed hydrologic system with canals, levees, and pumping stations. These structures have altered the hydrology of the Everglades ecosystem on both coastal and interior lands. Surface-water flows south of Lake Okeechobee have been regulated by an extensive canal network, begun in the 1940's, to provide for drainage, flood control, saltwater intrusion control, agricultural requirements, and various environmental needs. Much of the development and subsequent monitoring of canal and river discharge south of Lake Okeechobee has traditionally emphasized the eastern coastal areas of Florida. Recently, more emphasis has been placed on providing more accurate accounting of canal flows in the interior. The implementation and development of strategically placed streamflow and water-quality gaging sites in the interior will provide information for determining future surface-water flow requirements in the internal canal system. Subsequent studies based on accurate flow determinations from these sites will be used for computation of nutrient loadings in the internal canal system. Providing continuous-flow data at selected impact points for internal basins will complement 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.

The U.S. Geological Survey is presently conducting a study to accurately gage flows in canals entering and leaving Tribal Lands, the Big Cypress National Preserve, and WCA 3A. These flows will then be used in a multiagency effort to calculate nutrient loadings in the canals used by Native American Tribes and on Federal lands. Three real-time acoustic velocity flow weighted nutrient autosampler sites have been established, two on the L-28 and L-28 Intercepter canals at the southern border of the Seminole Tribe of Florida lands and one on the L-28 Intercepter canal where flows enter the western lands of the Miccosukee Tribe of Indians from the Big Cypress National Preserve. Acoustic Doppler Current Profiler calibration of the insitu acoustic velocity meter indexes has begun at all sites, and development of a technique utilizing the "sum of least squares regression" has begun on data for two of the three sites. Verification of the regression models has been limited by the availability of specific stage and velocity conditions. Velocity data collected during the dry season have shown a phenomenon known as acoustic refraction or ray bending produced by thermal stratification in the water column during extended periods of very slow flow. A South Florida Water Management District ancillary product of this effort will be a quality assurance template for future acoustic flow weighted nutrient sample station installation, in-situ calibration of acoustic equipment, and nutrient sample collection protocols based on weighting of flows.

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