The Okefenokee is a huge depression, shaped like a gentle saucer tilting south. Higher land lies to the north (the Bacon Terraces), the west (the Tifton Upland) and the East (Trail Ridge). Trail Ridge is probably a former barrier island built roughly one million years ago, when the sea level was about 40 miles further inland than it is today. The Okefenokee depression was a marshy lagoon behind the ridge. Over the millennia, the sea retreated. The clays of the old lagoon translocated through sands below them, helping to seal the bottom of the swamp. Water was unable to flow east, blocked by Trail Ridge, so the Suwanee River drained to the Gulf. (Eventually the St. Mary's River breached the ridge, allowing some of the Okefenokee's water to reach the Atlantic.)

For a time, the Okefenokee was not a swamp: it was probably a pine and oak forest with scattered wetlands and freshwater streams. Water tables rose, however, and large areas became flooded. Peat began to build about five to seven thousand years ago, forming the Okefenokee Swamp as we know it today.

One million years ago, Trail Ridge was a barrier island, like those of the Georgia Coast today. The Okefenokee was a marshy lagoon behind it. Photo by Hugh and Carol Nourse.

Peat

Peat is made up of partially decomposed plant material. It forms when there is not enough oxygen available for bacteria to break down dead vegetation. The lack of oxygen occurs because the gently tilted saucer beneath the swamp, with its clay seal and some impermeable rock beneath, holds in the water, which moves very slowly in the swamp. (This is especially true on the east side; Suwanee River currents move water faster on the west side of the swamp.) The oxygen in the slow moving water is quickly used up, and without oxygen, bacteria must break down organic material using slower, less efficient anaerobic (oxygen-free) processes. Swamp gas (methane) is a by-product. These slow decay processes fail to break down all the plant material, so the peat builds.

Contrary to what many believe, the Okefenokee is not a sphagnum moss bog. Peat cores from the swamp bottom show that the peat is mostly made up of the remains of the most common plants here, such as water lilies (Nymphaea odorata), Virginia chain fern (Woodwardia virginica), and pond cypress (Taxodium ascendens). Sphagnum mosses, which are the major ingredients of many northern bogs, are present in the Okefenokee, but they don't play such a large role here.

The peat is very acidic (the pH is roughly 3.5, with 7 being neutral). Humic acids and tannic acids seep from the soils of the nearby pine-oak uplands and from the decomposing peat. Sphagnum moss adds to the acidic conditions by acting like a "hydrogen pump", releasing hydrogen ions (which make the water acidic) while taking in and locking up other nutrients.

Exposed peat, graced by the splayed out parrot-beak shaped pitchers of a parrot pitcher plant (Sarracenia psitticina). Photo by Fred Mileshko.

Water Levels

Land in the Okefenokee rises and falls with the water level. Water level averages about two feet. Rain is the main source of water in the swamp. Since rainfall varies, huge changes in water level can take place from season to season and from year to year. The highest rain levels occur in the summer, but are offset by the draw down from evapotranspiration (evaporation combined with the uptake and expiration of water by plants).  Long summer droughts can expose huge expanses of peat to the sun, making it powder dry. Severe droughts occur about every 25 years.

Golden-club (Orontium aquaticum). Photo by Hugh and Carol Nourse.

A charred cypress tree, survivor of a surface fire. Photo by Richard Bryant from the book The Okefenokee Swamp.

Fire

Powder-dry peat is a prime fuel for fire. (In fact, peat was mined from the east side of the Okefenokee in the early 1900's as a fuel source.) Fire shapes a number of environments in Georgia, but it plays a particularly fascinating role in the Okefenokee. Here, fire not only burns the vegetation and changes the plant communities: at times, fire reshapes the land itself by searing through the peat. When peat is slightly moist, lightening strikes may cause surface fires that only kill the above ground parts of plants. But during droughts, the peat dries to deeper levels, and rapacious underground fires may burn for weeks, killing plants entirely and carving out new depressions. Such topography-changing fires apparently occur about every 100 to 150 years.

Peat blow-ups occur throughout the watery expanses of the prairies. A blow-up is a piece of peat that has been dislodged from the bottom of the swamp to float freely around the lake. Blow-ups appear when alligators and snapping turtles stir up the swamp floor and when pockets of methane gas lift pieces of peat to the surface, like air balloons.

The blow-up is not barren for long. The wind-carried seeds of broomsedge (Andropogon glomeratus), sedges (Carex spp.), some forbs, and beak rushes (Rhynchospora spp.) soon colonize the blow-ups, making tiny rafts of vegetation that float by your canoe. Once vegetated, the blow-ups are called batteries. Plant roots stabilize the peat, and dead vegetation builds it up, enlarging the battery.

Alligators rummaging through peat bottom of the swamp sometimes dislodge pieces to cause blow-ups. Photo by Richard Bryant.

The batteries float helter-skelter through the prairie, blocking boat trails, docking at other islands, growing and stabilizing as the plant roots spread. Eventually the roots anchor the mat to the peat floor of the prairie, and pond cypresses grow tall enough to tower over the shrubs. The battery is now called a house.

On left, blue flag iris (Iris virginica). Photo by Rita Collins. On right, progression from battery to house.