Florida
The Everglades appear brown in this NASA image, in contrast to the lighter urban colors of Miami on the eastern Florida coast.
The Florida Everglades are nicknamed the River of Grass. But this is no ordinary river, running in a channel between defined banks. Rather, it’s an entire region of slow-moving water wider than sixty miles, extending a hundred miles south from the Kissimmee River near Orlando to the Florida Bay. To identify the Everglades from the air, look for extensive wetlands (which can appear green to brownish green) west of Miami and south of the large, round Lake Okeechobee.
Southern Florida is dominated by a limestone shelf that was laid down between 70 and 25 million years ago, when this region was covered by a shallow sea. Over the last 50,000 years, changing sea levels have exposed and inundated this low-lying shelf numerous times. Limestone is formed when marine creatures with shells made of calcium carbonate pile up on the seafloor, mix with sand, and get compressed over time. At least five kinds of limestone are found in southern Florida, including Miami Limestone and limestone from the Fort Thompson Formation, which have the greatest influence on the Everglades.
Miami Limestone is made up of ooids: concentric shells of calcium carbonate that form around single grains of sand. Even after ooids are compressed into limestone, the rock is very porous and stores large quantities of groundwater, which sustain the Everglades during droughts and dry seasons. In contrast, the limestone of the Fort Thompson Formation is made of dense, hard layers that are much more compacted—to the point of impermeability in places. This encourages water to keep flowing.
Around 6000 years ago, the climate of south Florida started getting wetter, laying the foundation for the extensive marshes and wetlands we see today. As the River of Grass runs along this shelf of limestone, it drops only a few feet in elevation—about two inches per mile. Flowing south at an average rate of a half-mile per day, water may take months or even years to flow through the system from the Kissimmee River, through Lake Okeechobee, to the Florida Bay. Along the way, incremental drops in elevation give way to a variety of ecosystems, including sawgrass marshes, cypress swamps, hardwood hammocks, pinelands, mangrove islands and the marine habitats of Florida Bay. Such diversity of ecosystems is directly tied to the underlying geology of the region, which controls the flow of water along with the mineral and nutrient content of the soils.
The landform that we know today as south Florida has existed for about 17,000 years, with evidence of human habitation dating back to around 14,000 years ago, when the landscape was much drier and occupied by giant ground sloths and saber-toothed cats.
The River of Grass starts at Lake Okeechobee and runs south toward Florida Bay, along a giant shelf of limestone. Incremental drops support sawgrass marshes, cypress swamps, mangrove islands, and other ecosystems.
You might fly over the Florida Everglades en route to Miami or Key West, Florida. Look for extensive wetlands extending south from Lake Okeechobee.
