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South Florida's hydrologic systems

Hydrologic changes in southwest Florida

History of the Study
Regional System
Hydrologic Systems
- Importance of Water
- Aquifers
- Water Balance
> Hydrologic Changes
- Quantity Problems
- Resource Limits
- Quality
Final Word
PDF version
Before 1960 the hydrologic regimen of the Big Cypress Swamp watershed in southwest Florida remained relatively unaltered, except for minor changes brought on by shallow borrow canals used in road construction. Not until the 1960's did major changes begin in the west part of the watershed as a result of two primary canal systems, the Golden Gate Canal system and the Faka Union Canal system (fig. 1). Their purpose was to lower ground-water levels, make the land suitable for urbanization, and reduce flooding.

In a 195-km2 (75-mi2) area immediately inland from Naples, water levels were lowered 0.6 m (2 ft) or more over a span of 4 or 5 years as a result of construction of the Golden Gate Canal network (Klein, 1972). Before that area was drained, it was inundated during most of the rainy season and for 2 or 3 months afterward. Farther inland, to the east and southeast, the Faka Union Canal network, still under construction, has also lowered water levels (Carter and others, 1973), but the total areal extent and amount of lowering there cannot be evaluated until canal construction and water-control installations are completed. An indication of the change in water level and in the range in water- level fluctuations in the inland part of the Golden Gate Canal drainage area is shown by hydrograph in figure 24 (Klein and others, 1973). Levels have been lowered, as a result of increased movement of water from the interior toward the coast through the canals.

The original plan for development in southwest Florida called for controlled drainage, but the original plan in southeast Florida was one of uncontrolled drainage. In the southwest, canal flows are controlled by a series of weirs that reduced the possibility of saltwater intrusion. The weirs are set at progressively higher elevations inland, thereby stepping up water levels to simulate the predrainage hydraulic gradients but at lower elevations. According to McCoy (1972), the problem of seawater intrusion along the coastal areas is not magnified because water levels are not lowered excessively, as they were in southeast Florida. Maintaining water levels as high as possible is of utmost importance in the southwest Florida area because no provisions have been made for establishment of water conservation areas to store excess water for later distribution during the dry seasons, as has been done in the southeast.

Hydrologic information obtained in the Naples area at the end of the 1973-74 dry season showed that record low rainfall and prolonged pumping of municipal wells at high rates caused ground-water levels to be critically low and thus that any increase in withdrawal rates there could cause problems of seawater intrusion. These low levels and the projected increase in water demand emphasize the importance of keeping water levels high in the western part of the Big Cypress Swamp. Planning and water-supply agencies expect to develop water supplies to satisfy future urban demands from this part of the swamp.

The Golden Gate Canal system and the Faka Union Canal system are cut into the highly permeable limestone of the shallow aquifer. Because of the high permeability, ground water drains rapidly to the canals and thereby lowers the annual peak water levels. Wherever ponding occurs within those drainage areas during the rainy season, it is likely to be local and short lived. Thus, the pattern of slow, prolonged southward sheet flow of freshwater through the west part of the Big Cypress to the Gulf estuaries has been changed to one of accelerated and shortened-period runoff, primarily through the canal systems. Accelerated flow through canal systems tends to increase the opportunity for transport of pollutants and water of poor quality to the estuaries. In the eastern half of the Big Cypress Swamp, no drainage works have been constructed with outlets to the ocean, and therefore inundation occurs soon after the rainy season starts and continues for several weeks to months into the dry season. Natural surface flows of freshwater from the eastern Big Cypress area sustain estuarine conditions in the northwest part of Everglades National Park.

hydrograph of a well in the western Big Cypress Swamp, 1961-70, compared to rainfall for the same period

FIGURE 24. Hydrograph of a well in the western Big Cypress Swamp, 1961-70, compared to rainfall for the same period. [larger image]

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Last updated: 04 September, 2013 @ 02:04 PM (KP)