Effects of the Suwannee River Sill on the Okefenokee Swamp Hydrological Environment Cynthia S. Loftin USGS-BRD Maine Cooperative Fish and Wildlife Research Unit University of Maine, Orono Wiley M. Kitchens USGS-BRD Florida Cooperative Fish and Wildlife Research Unit University of Florida, Gainesville

Savannah Macon Atlanta Jacksonville Tallahassee Georgia Okefenokee National Wildlife Refuge

Project Objective: Has the Suwannee River sill affected the Okefenokee Swamp hydrological environment and vegetation distributions?

Water Control Structures Main Channel Suwannee River Sill

Forested Swamp Smilax spp. and shrubs Herbaceous Prairie Aquatic Prairie Upland Pine Forest

Sill Area Average Conditions Sill Area, Dry Conditions Main Channel, Suwannee River

Sill Ditch, Dry Conditions

Minute Hour Day Month Year 100 years 1000 years 10,000 years 10 years Duration of Disturbance or Landscape Component (Log Years) Space (Log Kilometers) 1cm1m1km1000km landscape forest stand patch canopy leaf Fire Thunderstorms Logging Drought Hydrology Change Hurricanes Processes Affecting the Okefenokee Landscape and the Scale of those Effects

Hydrological Features Physical Environment Vegetation History Species’ Hydrological Environment Manipulative Processes Comparisons: HYDRO-MODEL Map Overlays Species’ Models Landscape Patterns and Processes Suwannee Sill Effects

Weather, Water Level, Flow Topography, Peat Thickness Hydrological Features Physical Environment

Pre-logging Surveys 1977 CIR Photos 1952 BW Photos 1990 Imagery Vegetation History

Species’ Hydrological Environments

Suwannee Canal Logging 1950sPeat Mining Suwannee Sill Construction Manipulative Processes

Suwannee River Sill Flow Features and Directions During Average Conditions

JAN MAR MAY JULY SEPTNOV Month Water Depth (m) West Southeast Central Northeast East Daily Water Depths in Swamp Basins (1993)

Swamp Topographic Surface and Hydrologic Basins

Rain Inflow Outflow Evapotranspiration Water Depth Flow Direction Calculation Amount of Water to Move Water Movement Ground Surface Elevation/Roughness + HYDRO-MODEL Steps

Model Simulations Northeast West no sill with sill no outflow 75% ET 150% ET Water Depth (m) Year

SCRA SCFSP Year Water Depth (m) Annual Average Water Depth

Inflow Outflow Sill ET Sill (upper basin) Outflow ET Areas Affected by Model Manipulations

Areas Affected by Sill Operation Flow direction reversal in very high water level conditions Extended hydroperiod in high water conditions Reduced hydroperiod Extended hydroperiod in low water conditions Increased outflow in very high water level conditions

Year Percent Vegetation Composition of Okefenokee National Wildlife Refuge Vegetation Type in 1990 Regional Comparison of Changes in Vegetation Types, Before and After Sill Construction Upland Pine Wet Forest Shrub Prairie

Area (ha) Percent Proportion and Size of Wildfires by 5-Year Intervals ( ) 5-Year Interval Sill Construction ~ ~

Prairie Prairie- Shrub- Cypress Shrub-Cypress-Swamp Cypress- Bay- Blackgum Swamp Mixed Blackgum Swamp Shrub-Swamp Prairie-Shrub-Swamp Bay-Shrub-Swamp Bay-Swamp no fire moderate fire severe fire + logging Vegetation Succession in Okefenokee Swamp

Summary Points The Suwannee River sill is increasing flooding depth and duration in 18% of the refuge, primarily during periods of abundant precipitation. Although fire management may be affecting fire distributions, the sill has little effect of the swamp fire regime.

Summary Points (continued) The observed changes result from logging, fire management, hydrologic manipulations, and vegetation succession. The role of these processes as the primary driver of change is influenced by the location within the swamp.

Acknowledgements Okefenokee National Wildlife Refuge, USFWS-Region 4, USGS-BRD, N. Ansay, J. Aufmuth, J. Brookshire, C. Depkin, D. Evenson, J. Kitchens, J. Loftin, D. O’Neill, P. Owen, L. Pearlstine, B. Rasberry, K. Williges