The Everglades are a subtropical wetland of sawgrass marsh, sloughs, tree islands and mangrove forest in southern Florida, United States. Water spilling south from Lake Okeechobee spreads into a shallow sheet up to 97 km (60 mi) wide and more than 160 km (100 mi) long, creeping across a limestone shelf to Florida Bay, which is why Marjory Stoneman Douglas called it a "River of Grass" in her 1947 book. About half of the original wetland has been turned into farms and cities. The southern 20 percent is protected in Everglades National Park, 6,107 km² (2,358 sq mi), which became a UNESCO Biosphere Reserve in 1976, a World Heritage Site in 1979 and a Ramsar Wetland of International Importance in 1987. Everything in the Everglades runs on a two-season year: a dry winter from November to April and a wet summer from May to October.
November to April: the River of Grass slows down
The Everglades water year is best started in November, when the summer storms stop and the marsh begins to drain. The slope from Lake Okeechobee to Florida Bay is only about 5 cm per 1.6 km (2 inches per mile), so even in a wet year the sheet of water moves somewhere between 0.4 and 0.8 km (a quarter to half a mile) a day, depending on the measurement. Water leaving the lake can take months or years to reach Florida Bay. Much of what falls is stored in the permeable limestone under the marsh, chiefly the Miami Limestone and the Fort Thompson Formation, and that stored water recharges the Biscayne Aquifer that supplies Miami.
Winter at Royal Palm, the ranger station near Homestead, is mild and dry. The 1991–2020 normals give a January mean of 19.2 °C (66.6 °F), and December to March each bring less than 50 mm of rain. Through these months the water drops, first off the wet prairies, which are flooded for only three to seven months a year and hold about 10 cm (4 in) of water when they are wet. The longer-flooded ground keeps its water and its peat: places under water for more than nine months a year build peat, while places flooded for six months or less are floored with grey marl, the dried remains of periphyton mats.
Alligator holes and wood stork colonies in the shrinking marsh
As the sheet of water breaks into pools, the American alligator becomes the marsh's water engineer. Using claws and snouts, alligators dig out low spots in the wet prairies and keep them clear of vegetation, so the hollows stay flooded through the dry season. Fish, turtles, aquatic invertebrates, small mammals and birds crowd into these alligator holes during drought, and the alligators eat some of the visitors. Solution holes, pits where acidic water has dissolved the limestone, do the same job on a smaller scale, sheltering crayfish, snails and tadpoles that feed young wading birds.
The falling water is also the signal for nesting. The wood stork, the only stork that breeds in North America, starts breeding when water levels drop and fish become concentrated, because in the dry season it forages by walking slowly with its bill submerged, groping for prey in water that averages about 17 cm deep. An adult needs about 520 g of food a day, and a nesting family about 200 kg over a breeding season, so a wet winter that keeps fish dispersed can make a whole colony fail. The 1999 federal "Restudy" listed harmful timing of water releases from canals and pumping stations among its signs of damage, alongside an 85 to 90 percent fall in wading bird numbers over the previous 50 years. The wood stork was listed as endangered in the United States because of habitat loss in the Everglades, its old stronghold, and was later downlisted to threatened as its range spread north.
Winter is also when saltwater moves in. With little fresh water pouring out to Florida Bay, the sea creeps inland into the coastal prairie, the buffer between mangroves and freshwater marsh, and in long droughts mangroves begin to grow in places that were fresh. December to March is the park's busiest visitor season, partly because the mosquitoes are at their least active, and the Anhinga Trail near the Royal Palm Visitor Center, a short paved path and boardwalk over the sawgrass marsh of Taylor Slough, is where crowds watch alligators and wading birds gathered around the last deep water.
Peat fires, pine rockland and the dry spring
April and May are the driest and most flammable weeks of the year, and fire is part of the Everglades as much as water is. Most natural fires are started by lightning, and in the wet season they mostly burn sawgrass above the waterline, leaving the roots alive under water. A dry-season fire is different: it can ignite the organic peat and burn down through root systems. Peat builds up at only about 30 cm (1 ft) every 225 years, and charcoal layers show that parts of the Everglades went through years of severe burning, most recently around 940 BC.
Drainage made the dry season far worse. During the drought of the 1930s, with the new dike holding water in Lake Okeechobee and canals draining what was left, the peat turned to dust, and in 1939 about 4,000 km² (a million acres) of the Everglades burned under smoke that hung over Miami. Exposed peat also oxidised, and in some places about 2.4 m (8 ft) of soil was lost. The highest ground in the park, the pine rockland of Long Pine Key, needs fire: South Florida slash pine has insulating bark, and without burning the pine forest turns into hardwood hammock. The park protects about 80 km² (19,840 acres) of pine rockland and sets prescribed fires there every three to seven years.
May to October: 70 percent of the rain in six months
The wet season arrives with the Bermuda High. Moist easterly air heats over the land, rises, and breaks into thunderstorms whose average time of day across South Florida is 2 pm. About 70 percent of the region's yearly rainfall falls between May and October. At Royal Palm, June and September each average about 229 mm, against 43 mm in January, and the year totals about 1,457 mm (57 in). Rain is not the only flow: evapotranspiration, the water lifted back into the air by sun and plants, removes about 100 cm (40 in) a year in a normal year and more than 130 cm in a drought, sometimes more than falls as rain.
The rising water spreads back over the marl prairies and refills the sloughs, and the fish that survived in alligator holes disperse across the marsh to breed. Sawgrass grows toward the coast as fresh water pushes the salt back out into Florida Bay and the Ten Thousand Islands, a mangrove estuary of almost 810 km² on the Gulf coast. Hardwood hammocks, tree islands raised 0.3 to 0.9 m above the sloughs, stand above the flood with moats of moving water around them; gumbo limbo, live oak and royal palm grow there, mostly no taller than about 17 m. Summer is hot: Royal Palm's August mean is 28.6 °C (83.5 °F), and inland highs usually pass 32 °C.
August and September: storms from Donna to Irma
Rainfall peaks late in the wet season, in August and September, when tropical depressions add to the daily storms. On average a tropical storm affects South Florida about once a year and a major hurricane about once a decade; between 1871 and 1981, 138 tropical cyclones passed directly over or close to the Everglades. Storms spread seeds and renew mangroves, and the marsh usually recovers: Hurricane Donna in 1960 left miles of mangrove as standing dead snags, and 30 years later the area had fully regrown.
People recover less easily. Hurricanes in 1926 and 1928 pushed Lake Okeechobee through its levees and killed thousands of people around the lake. The response was the Herbert Hoover Dike, 106 km (66 mi) long, built around the southern shore between 1930 and 1937. In 2005 Katrina, Rita and Wilma all hit the park; the lodge at Flamingo, battered by 201 km/h (125 mph) winds and a 2.4 m (8 ft) storm surge, was demolished in 2009. Hurricane Irma in 2017 damaged the coastal mangroves, hit Flamingo again and destroyed the original Gulf Coast Visitor Center.
The engineered year: canals, pumps and the Tamiami Trail
Since 1947, the natural calendar has been overlaid by an engineered one. That year Congress created the Central and Southern Florida Flood Control Project, which built about 2,300 km (1,400 mi) of canals, levees and water-control structures. The lake was held between legal limits of 4.3 and 5.2 m (14 and 17 ft), and the Tamiami Trail, the Tampa–Miami road built across the marsh in the 1920s, blocked the flow into the southern Everglades. Water now arrives when gates open, not when rain falls; the park's own visitor brochure describes the Everglades as "on life support". UNESCO placed the park on its List of World Heritage in Danger from 1993 to 2007, and again in 2010.
The Comprehensive Everglades Restoration Plan, signed by President Bill Clinton on 11 December 2000 with a $7.8 billion estimate, aims to store water that would otherwise be dumped to the sea and release it on a more natural schedule. Part of that is lifting the Tamiami Trail: work on a 1.6 km (1 mi) bridge began in December 2009, and a further 8.9 km (5.5 mi) of bridging was proposed in May 2010 so water could flow south again. The calendar has new pressures too. More than 2,000 Burmese pythons had been removed from the park by 2017; in a 2015 study, pythons accounted for 77 percent of the predation deaths among marsh rabbits released into python country. Rising sea level is pushing salt into freshwater marsh at the coast, and the snail kite, which eats apple snails that need steady water levels, was graded F (near irreversible damage) in a 2012 National Research Council review.
For visitor routes through all of these seasons, from Shark Valley to Flamingo, see our Everglades National Park guide. For how the limestone peninsula beneath the marsh came to be, read about the geography of the Florida peninsula, and for a peat swamp that runs on a very different water year about 600 km to the north, compare the Okefenokee Swamp in Georgia.