The Everglades is a vast, subtropical wetland system covering much of the southern tip of the Florida peninsula. Often described as the “River of Grass,” this immense landscape is a wide, shallow, and slow-moving sheet of water flowing over a nearly flat, limestone base. Its formation resulted from a combination of deep geological history, a massive northern water source, and an extremely subtle geographic slope. Understanding its formation requires looking beneath the sawgrass to the ancient rock that holds and directs the water.
The Ancient Bedrock Foundation
The foundation of the Everglades began forming millions of years ago, with the most recent structure laid down during the Pleistocene epoch due to fluctuating global sea levels. When seas rose during warm periods, they covered the Florida platform, depositing marine sediments. This process created the porous carbonate rocks that form the bedrock, primarily the Tamiami Formation and the younger Miami Limestone.
The Tamiami Formation underlies the Big Cypress Swamp to the west, while the Miami Limestone forms the Atlantic Coastal Ridge and the base beneath the northeastern Everglades. This highly porous limestone acts like a sponge, preventing water from draining deeply away from the surface. The overall flatness of this shelf dictates the shallow nature of the overlying wetland system and created the necessary “container” for the River of Grass.
The Water Engine: Lake Okeechobee and the Kissimmee River
The water sustaining the Everglades originates far north in the Kissimmee River Basin near Orlando. The Kissimmee River and its tributaries drain a 3,000-square-mile watershed south into Lake Okeechobee, forming the headwaters of the system. The river is the largest single source, contributing over 60 percent of the water flowing into the lake.
Lake Okeechobee, the second-largest freshwater lake in the United States, acts as the massive, shallow reservoir. Historically, during the wet season, the lake overflowed its southern rim, initiating the southward flow. This overflow, combined with substantial local rainfall (averaging 60 inches annually), provides the continuous supply needed to flood the low-lying peninsula and allowed the wetland to extend over 100 miles to Florida Bay.
The Defining Process: Slow Sheet Flow
The mechanism defining the Everglades is the process of slow sheet flow, or the “River of Grass.” The landscape between Lake Okeechobee and the Gulf of Mexico is remarkably flat, dropping only 12 to 14 feet over roughly 100 miles. This results in an incredibly shallow slope, sometimes less than two inches per mile.
Due to this subtle slope and the wide, flat limestone bedrock, water does not channelize into a conventional river. Instead, it spreads across the entire peninsula, moving almost imperceptibly as a sheet, traveling only a few dozen feet per day. This constant, slow-moving, shallow water led to the accumulation of organic material from decaying plant life. This formed thick layers of peat and muck soils over the limestone base, a direct result of the long-term saturation provided by the sheet flow.
Ecosystem Development and Unique Habitats
The unusual hydrological conditions created by the sheet flow resulted in a mosaic of distinct habitats. The dominant plant life across the open marsh is sawgrass, a sharp-edged sedge that thrives in the consistently wet, nutrient-poor peat and muck soils. The alternating pattern of slightly higher sawgrass ridges and deeper, open-water sloughs is maintained by the direction of the sheet flow.
Slight variations in elevation, often just a few inches, dramatically affect the local ecosystem. These minor high points support tree islands—elevated hammocks of tropical hardwood trees that rise above the surrounding marsh. These islands are often teardrop-shaped, with the rounded end pointing north, as the slow current has shaped their outline over millennia. Cypress swamps, featuring water-adapted trees like the bald cypress, occur in areas with longer hydroperiods, forming domes where the deepest water is at the center.