Substantial deviations from the normal dry-season regime occur as a result of the El Niño phenomenon, which occurs sporadically every 3–7 years and lasts 1–2 or more years, which bring abnormally wet and powerful extra-tropical cyclones across the southern United States, including Florida, during the winter–spring “dry” season. El Niño is also associated with reduced hurricane frequency during the summer wet season because of high-altitude easterly winds that often shear off the tops of developing hurricanes. The periodic opposite extreme, called “La Niña,” reverses these effects.
Map courtesy of the South Florida Water Management District
Before changes began in 1882, the entire ecosystem that included the Everglades was a watershed beginning near the present location of Orlando: the Kissimmee–Lake Okeechobee–Everglades watershed, sometimes given the acronym KLOE. Flows from the Kissimmee River and several smaller streams ended in Lake Okeechobee, from which there was no outlet during the dry season. But in normal summer rainy seasons, the water level of Lake Okeechobee rose, and upon reaching a stage of about 19 to 20 feet (above sea level), the lake merged into the Everglades with flows across its southern rim through a forest of pond apple, also called custard apple. Early explorers tried to find navigable outlets from the lake through streams leading into this swamp, which fringed the south and southeastern rim of the lake like a necklace up to 3 miles thick. All of these streams ended in a profusion of swamp forest or dense sawgrass. Beyond the ends of the short streams lay the tall, nearly impenetrable sawgrass of the northern Everglades. Sawgrass dominated the northern Everglades interior but was far from the only plant community of this vast ecosystem, which was 40 miles wide and extended almost 100 miles to the tidal waters of Florida Bay and the Gulf of Mexico. The northern Everglades “sawgrass plain” extended about 15 miles south and southeast from the pond apple swamp before giving way to a more varied landscape. Along the edges of the Everglades were more open, wet prairies that harbored a rich variety of marsh plants. Through the interior, there were deeper open-water sloughs (pronounced “slews”) with surfaces adorned by lily pads. Sloughs were bordered by a foot and a half higher “ridges” of sawgrass, and the landscape was dotted with further elevated tree islands. This mosaic covered the major portion of the Everglades, where the elongated shapes of sloughs, sawgrass ridges, and tree islands were conspicuously aligned with the southward flow of water.
Although sloughs were easy to traverse in canoes, there were no open channels through the Everglades. The water moved in a vast “sheet,” with flows perceptible to the patient, steady eye through the relatively open sloughs and, even more slowly, through sawgrass. The slope from Lake Okeechobee to tidal waters averaged less than 3 inches per mile. With higher terrain in the Big Cypress Swamp to the west and the Atlantic Coastal Ridge to the east (its highest elevations scarcely over 20 feet), the Everglades waters were guided slowly southward by the imperceptible slope of the terrain. The rate of flow, slowed by the marsh vegetation, has been measured to range from negligible to about 10 feet per minute, but averages less than a foot per minute. The depth varied considerably with seasons and years, but rarely exceeded 4 feet in the deepest marshes. Average wet-season maximum depths in the various marsh plant communities ranged between 1 and 3 feet. Throughout the entire ecosystem, the flat terrain and seasonally abundant water made small variations in topography important. Differences of a few inches in elevation cause large responses in vegetation. Ranging from freshwater marshes and tree island communities to cypress, pinelands, and upland forests, and finally to saline environments of the coast, these variations added intriguing dimensions to the landscapes of the region.