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Water Quality in Big Cypress National Preserve and Everglades National Park - Trends and Spatial Characteristics of Selected Constituents

WRI 03-4249

Trends in Rainfall, Water Levels and Flows

Water quality in the BICY and EVER is affected by seasonal and long-term changes in rainfall (fig. 3), water levels, and flows. During the period when water-quality data are available, annual water conditions in the Everglades have been described as being dry in 1974-76 and 1985, and very dry in 1989-91 (Frederick and Ogden, 2001). Low water levels in the marshes and sloughs generally result in ponding and increased major ion and nutrient concentrations because of the enhanced breakdown of organic material and the build-up of wastes from aquatic and terrestrial wildlife that concentrate in and near the remaining surface water. Conversely, high water levels and flowing water may decrease concentrations by dilution or flushing major ions and nutrients out of the marsh, or may increase concentrations by introducing water enriched in major ions and nutrients from agricultural or urban sources. Tropical storms and hurricanes might be expected to affect water quality because of heavy rainfall and high winds, but it appears that such effects are minimal, at least in remote regions of EVER, where little change was seen in water quality after Hurricane Andrew passed over the park on August 24, 1992 (Roman and others, 1994).

Graph showing average annual rainfall at 20 sites in south Florida.

Figure 3. Average annual rainfall at 20 sites in south Florida. [larger image]

Water levels and flows in the eastern Everglades have been altered by development and water management. The eastern Everglades (including the eastern part of Everglades National Park) encompasses a vegetative region know as the southern marl-forming marsh (Davis and others, 1994) - an area of rugged limestone at the surface (Miami Rock Ridge), marl marshes and prairies, mangrove-lined creeks near the coast, and a few deeper water sloughs, including Taylor Slough. Annual fluctuations in water levels were dampened in the 1970's (fig. 4) by changes in water management, primarily the construction of the L-31 canal and later the C-111 canal system.

Graph showing average monthly water elevation in feet above NGVD 29 at wells S-196 and S-196A near Homestead, FL

Figure 4. Average monthly water elevation in feet above NGVD 29 at wells S-196 and S-196A near Homestead, FL. [larger image]

Shark River Slough, the major drainage feature in the central Everglades, lies to the west of the marl-forming marsh of the eastern Everglades. The Slough originally extended about 100 miles in a southwesterly direction and drained into the mangrove forests and Ponce de Leon Bay of southwest Florida. The Tamiami Trail (US 41), constructed across the Slough in the 1920's, was the first impediment to the Slough's flow. Drainage and impoundment to the north of the Trail in the 1960's and 70's further isolated the Slough from its headwaters in the central Everglades. The L-67 canals, which were dug in the early 1960's, brought water from the northern Everglades to the newly constructed S-12 structures (S-12s), where waters flowed into EVER along the western side of the Slough. In 1966-67, the L-67 canal was extended south from the Tamiami Trail along what was then the eastern boundary of the park and conveyed waters into the Slough 9.5 miles south of the Trail. In the early 1980's, the gates of the S-12s were left open, so waters flowed to EVER based on hydraulic gradient. In 1985, a Rainfall Plan was initiated so that water flowed to the park following a more natural pattern that reflected upstream rainfall. Compared to the high rainfall and flows in 1969-70, annual flows to EVER across the Tamiami Trail were low during the 1970's and 1980's (fig. 5). The years 1989 and 1990 were dry with little flow to EVER. Annual flows increased in the 1990's and peaked in 1996 (fig. 5). Also during the 1990's, proportionally more water passed through the eastern section of the Tamiami Trail (L-30 to L-67) into the Shark River Slough than in earlier years. Efforts currently are underway to divert even more water to the eastern part of the Slough to more closely mimic natural (predevelopment) flow patterns.

Graph showing average annual discharge under the Tamiami Trail. L-30 to L-67A; S-12s, 40-Mile Bend to Monroe, and Monroe to Carnestown, and Barron River

Figure 5. Average annual discharge under the Tamiami Trail. L-30 to L-67A; S-12s, 40-Mile Bend to Monroe, and Monroe to Carnestown, and Barron River. [Discharge measurements started during the 1960's in some sections.] [larger image]

West of Shark River Slough lies the slightly higher lands of the western Everglades and the Big Cypress Swamp. These lands are a mosaic of cypress strands and sloughs, rocky marshes, and slightly higher pine forests. Disruption of natural flows has been minimal in the Swamp compared with the effects on Shark River Slough and the eastern Everglades. The Tamiami Trail, constructed in the 1920's, is regarded as having less effect on flows in BICY than in EVER. Canals in the Big Cypress Swamp are small and have less effect on hydrology than the larger canals in the Everglades. Water in the Big Cypress Swamp flows under the Trail through numerous culverts and bridges. Flows in the Big Cypress Swamp have been affected by the L-28 Interceptor Canal drainage system (fig. 1), which was constructed in the mid-1960's and blocked flows to the swamp from the Everglades, and by the Barron River Canal (fig. 1), which was completed in 1926 and intercepted flows from the Okaloacoochee Slough and Deep Lake Strand and discharged water to the coast near Everglades City. Structures (S-343A, S-343B, and S-344) were cut in the L-28 levee in 1983-85 to allow water to flow from the western Everglades (WCA 3A) into the eastern Big Cypress Swamp. Flows in the section of Tamiami Trail from 40-Mile Bend to Monroe (fig. 1) increased markedly in the mid-1990's, as a result of abundant rainfall and the construction of the S-343A, S-343B, and S-344 water control structures (fig. 5).

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