Okefenokee Swamp Restoration

Effect of the Suwannee River Sill on the hydrology and ecology of the Okefenokee Swamp

The Okefenokee Swamp is one of the largest freshwater wetlands in the world. Currently protected and managed as a national wildlife refuge and national wilderness area, the swamp has a history of human manipulation and modification. The swamp landscape is dynamic. Vegetation compositions and distributions continually change as the hydrologic environment changes. The dynamics are driven by natural processes such as peat accumulation and wildfire, as well as the artificial manipulations of the recent past. My graduate student (Cynthia Loftin, University of Florida, graduated) and I have conducted research on this system since early 1993. The project has been conducted in several phases as follows:

(a).Hydrology Studies. A spatial hydrology model (HYDRO-MODEL) was developed to describe the swamp hydrologic environment, identify local- and system-level sensitivities to potential alterations in the Suwannee River sill structure, and compare type, location, and extent of hydrologic and vegetation changes. The model was developed and calibrated with weather and vegetation data representing conditions in the Okefenokee National Wildlife Refuge between 1980-1993, and topographic information collected during 1991-1994. Output data include water depth, water surface elevation, and amount of water moved in each time step. Changes in relationships between the sill area water depths and stations throughout the swamp under high, average, and low water level conditions were assessed by comparing slopes of regression relationships and coefficients of variation. These assessments provided clues to the spatial and temporal extent of the sill's effects, and variability of these effects with overall water level conditions.

(b). Wildfires in the Suwannee River Sill Affected Area. The primary purposes of the Suwannee River Sill were to facilitate wildfire control by creating impounded conditions during periods of drought, and to arrest the spread of wildfires across the landscape by prolonging inundation. By comparing maps of wildfire ignition location and burn extent with a delineation of the sill-affected area and information on general hydrologic conditions at the time of the wildfires, we were able to determine that the sill was not elevating water levels during the period of high fire frequency, nor was the number of ignitions suppressed in the area of the sill's influence.

(c). Vegetation Change in the Suwannee River Sill Affected Area. The Okefenokee Swamp vegetation landscape is dynamic. Fluctuations in species compositions and distributions may be the consequences of naturally occurring community succession, and may also result from historic logging, wildfire management, or manipulations of the landscape hydrology. Areas of vegetation change attributed to hydrologic modifications of the Suwannee River Sill were compared with ERDAS-IMAGINE summary overlays of the estimated sill impact area. Proportions of vegetation types inside and outside of the affected area were estimated and compared between the areas with these overlays. In addition, analyses of species associations along hydrologic gradients were conducted for standing vegetation and the seed bank contents. Using multiple regression techniques, a response surface describing the relationship between specific species and gradients of depth inundation and hydroperiod was developed to predict future vegetative landscapes resulting from manipulation of the sill, including its removal. This project was conducted under my direction as principal investigator and graduate supervisor to Dr. Loftin. The studies were funded by the Fish and Wildlife Service.