The geologic history of the Okefenokee Swamp is the story of the dynamics of the Atlantic Coastal Plain. To understand the Atlantic Coastal Plain, one must understand that relative sea levels have been highly variable over geologic time scales, and all of the Atlantic Coastal Plain has been beneath the ocean at least several times.
There is a large limestone aquifer system underlying the Atlantic Coastal Plain. Limestone is formed from the calcium carbonate-rich skeletons and shells of diatoms and other ocean creatures. When ocean creatures die, their shells and skeletons fall to the ocean floor. Eventually time and pressure transform this accumulation of calcium carbonate into limestone rock—which is highly porous and makes productive aquifers. The productive limestone aquifer underlying the Georgia and North Florida Coastal Plain is called the Floridan aquifer.
Over much of the Coastal Plain, and underneath the Okefenokee Swamp, the Floridan aquifer is capped by the Hawthorne Formation, a low permeability layer of highly weathered limestone and clays that acts to partly separate the Floridan aquifer below from the surficial aquifer above. The low permeability of the Hawthorne formation keeps the Okefenokee Swamp from simply draining downwards into the Floridan aquifer. The Hawthorne isn’t impermeable, however, and some amount of Okefenokee water does percolate downward to the Floridan.
If you look at a topographic or physiographic map of Georgia, you see long low ridges running parallel to the coast that look like barrier islands, except they are tens of miles from the ocean. These are in fact either old barrier islands formed when sea-levels were higher or wind-deposited (aeolian) dunes, and now they are long, low ridges of sand, clays, and organic matter. The longest and highest of these ridges is called Trail Ridge, and it runs about 160km from Starke, FL to Hoboken, GA. Trail Ridge is bifurcated where the St. Marys River runs through it. The Okefenokee Swamp occurs in what may have been an ancient lagoon on the west side of Trail Ridge. Estimated ages of Trail Ridge range from as little as 45,000 years (late Pleistocene epoch) to 2.6 Million years (late Pliocene epoch).
Map of the current (Holocene) and past Pleistocene Coastlines of the Georgia Coast. From Hails and Hoyt, 1969.
West to east topographic and geologic profile of the Okefenokee Swamp showing how Trail Ridge blocks the west-to-east surface flow of water. The St. Marys River starts in the swamp, flows south and then east through a gap in Trail Ridge, and then back to the north along the east side of Trail Ridge as shown here. From USFWS 2006.
The Okefenokee exists in its present state for several connected reasons:
- Trail Ridge acts as an earthen dam blocking the natural drainage from west to east,
- the Hawthorne formation moves water slowly enough that the swamp waters don’t quickly drain to the Floridan aquifer,
- the continued accumulation of organic matter and formation of peat within the swamp further impede vertical percolation of water, and
- increased rainfall and the back-water effect of higher sea levels following the last continental glaciation created hydrologic conditions favorable to swamp formation.
The present incarnation of the swamp is surprisingly only about 6,500 years old (determined by radiocarbon dating of the deepest peat deposits), and it has changed continuously during that time as peat has accumulated within the swamp. Due to the topography and geology of the swamp, a swamp would form here anytime climatic and associated water table conditions were favorable. At the end of the last period of continental glaciation, the area’s climate was much drier, and water tables were lower. Local and regional geologic evidence suggests there was no Okefenokee Swamp in the period from 6,500 to over 10-12,000 years ago when the continental glaciers melted.
The above references information found in:
Parrish, F.K. and Rykiel Jr, E.J., 1979. Okefenokee Swamp origin: review and reconsideration. Journal of the Elisha Mitchell Scientific Society, pp.17-31.
Cohen, A.D., 1973. Petrology of some Holocene peat sediments from the Okefenokee swamp-marsh complex of southern Georgia. Geological Society of America Bulletin, 84(12), pp.3867-3878.
Hails, J.R. and Hoyt, J.H., 1969. An appraisal of the evolution of the lower Atlantic Coastal Plain of Georgia, USA. Transactions of the Institute of British Geographers, pp.53-68.
U.S. Fish and Wildlife Service. 2006. Comprehensive Conservation Plan for Okefenokee National Wildlife Refuge. Accessed 3 April 2013. Available from: http://catalog.data.gov/dataset/okefenokee-national-wildlife-refuge-comprehensiveconservation-plan
The hydrology of the swamp is largely vertical, rather than lateral, meaning the dominant inputs and outputs are rainfall and evapotranspiration. Rainfall accounts for approximately 75% of the input to the swamp, with the small streams to the northwest providing the remainder (see map). The swamp comprises just over half its own watershed, so rain falling on the swamp itself is the largest input to the swamp. Most water leaves the swamp as evaporation and transpiration (nearly 80%), with most of the rest draining to Suwannee and the St Marys Rivers. A small and unquantified amount of water percolates downward from the swamp to the Floridan aquifer.
Okefenokee Swamp Watershed – the swamp itself comprises 51% of the watershed. The swamp is fed mostly by rainfall and also by small streams flowing from the northwest. The swamp drains to both the Suwannee and St Marys Rivers. From Parrish and Rykiel 1979.
As a rain-fed system with minimal groundwater contribution, the swamp is highly sensitive to drought. During dry periods, water levels drop and the rivers quit flowing. When this happens, the swamp margins and islands become very susceptible to wildfire, and over the last century several large fires have burned large areas within the swamp and in the forest lands surrounding the swamp. Consequently, the swamp ecosystem is fire-adapted (see the Ecology section).
Fires from the swamp can and have caused large economic damages to the surrounding commercial forest lands. Most of the Swamp is designated Federal Wilderness Area without road access, so fighting fires within the swamp is difficult. Mitigation of fire risk is therefore focused on reducing fuel loads on the swamp margins and coordinating fire-fighting efforts among federal agencies, the Georgia Forestry Commission, the Florida Division of Forestry and private forest landowners, all of whom have joined together to form the Greater Okefenokee Association of Landowners (GOAL) to coordinate fire risk reduction.
After large and severe drought-fires of 1954 and 1955, the Federal government in 1960 constructed the Suwannee River sill—essentially a five-mile-long low dam meant to back up the Suwannee River outlet from the swamp, raising water levels in the swamp, and reducing the severity of droughts. Decades of observations and research showed that the backwater effect of the sill was small—affecting only about 1% of swamp area, and thus it was not effective in reducing the fire danger from drought. However, the sill had unintentionally developed a recreational purpose – it was useful for locals to access the swamp and fish the river, so the sill remains although it is no longer managed to hold back swamp waters.
Disturbance and constant change are key aspects of what makes the Okefenokee swamp ecosystem so diverse and important. Droughts and floods change water levels, fires scorch vegetation, and peat layers shift up and down, disturbing trees and shrubs. These changes make the Okefenokee into a mosaic landscape, a sort of patchwork of many different habitats. Each habitat has its own dominant vegetation. For example, drier, upland areas support mixed hardwood forest, longleaf and loblolly pine forests, and dry prairies. Where water is dominant or flooding is common, the Okefenokee supports sedge prairies, baldcypress swamps, floating peat bogs, sand islands and blackwater lakes. This patchwork means that the Okefenokee supports a great variety of plant and animal species. This is not only because the variety in habitats provides shelter and food for many species, but also because having multiple habitats close together supports those species that use different habitat types throughout their lives.
With so many different resources, habitats and micro-environments close at hand, the Okefenokee boasts a fantastic diversity of wildlife. This includes over 850 plant species and 400 vertebrate animals, not to mention countless invertebrates like insects, arachnids and micro-organisms. Vertebrates include more than 230 species of birds, 37 amphibian species, 66 reptile species, 48 mammal species and 36 fishes from 14 different families. Among these many animals that call the Okefenokee home, some are important endangered or threatened species like the Eastern indigo snake (Drymarchon couperi), Wood stork (Mycteria americana), Gopher tortoise (Gopherus polyphemus), and Red-cockaded woodpecker (Leuconotopicus borealis). The Okefenokee provides key habitat for these species, which have lost much of their former range to development. The Okefenokee’s large extent of riparian (riverside) shrub and forest habitat also provides valuable feeding habitat for neotropical migratory songbirds, many of which are in decline.
As a blackwater swamp, the Okefenokee’s waters are tea-brown and contain lots of dissolved organic matter. These dark, acidic waters keep nutrients and organic chemicals, the necessary building blocks for life, bound up in a dissolved state. This in turn can make nutrients hard to access for organisms living inside the swamp. The types of disturbance mentioned above (fires, floods, droughts, fluctuations in peat elevation) can occasionally free up access to nutrients. These fluctuations in the availability of key elements for life add to the constant change and variety of the Okefenokee.
Many plants and animals of the Okefenokee are specially adapted to deal with the swamp’s unique and sometimes challenging conditions. For example, several species of carnivorous plants thrive within the swamp. By capturing and digesting animal prey like small insects and micro-organisms, these plants gain nutrients that might be hard to pull from the soil or water. Plants like sundews (Drosera), butterworts (Pinguicula), pitcher plants (Nepenthes), and bladderworts (Utricularia) all use this strategy. Bladderworts are a particularly fascinating example, because their complex under-water structure provides a home for a diverse mini-ecosystem living in their roots and branches. These tiny ecosystems support rotigers, algae, crustaceans, insects, protists
, and a variety of worms, many of which the plant traps and digests for nutrients.
Beyond micro-ecosystems, the Okefenokee’s large size and many habitats also provide an ideal landscape for wildlife that require multiple habitats and resources over the course of their lives. Many salamanders and frogs move between sandy upland habitats and the swamps. Gray and red foxes, as well as black bear, skunks, opossums, raccoons, deer and bobcats move freely through the swamp, hunting food in wetter areas and denning in drier ones. Animals as big as back bears can shelter inside of the larger trees provided by the swamp’s old-growth patches of forests, the areas that were never clear-cut. A great variety of wading birds roost in the swamp’s tall trees at night, or nest in large colonies called rookeries, and move through the flooded areas to feed. The swamp is also an important wild refueling and resting spot for migratory birds. This includes both waterbirds, attracted to the Okefenokee’s waters, and songbirds like warblers, thrushes
, and vireos that can tank up on the great abundance of insects and other invertebrates produced by this dynamic ecosystem.