The U.S. Forest Service and the U.S. Forest Service and the U.S. Geological Survey are investigating the ecosystem dynamics of bottomland hardwood forests in the Southeastern United States. One of the study sites is located on the floodplain of the Coosawhatchie River, a fourth order, anastomosing river located in Allendale, Hampton, and Jasper Counties of Southeastern South Carolina. Ground- and surface-water levels are being monitored at the study site because the depth and duration of surface inundation and the elevation of the water table are important factors controlling vegetation community dynamics at the site. An analysis of the relation between groundwater and surface water is being made to better understand the factors controlling the hydrologic conditions at the begetation plots.

The drainage area of the Coosawhatchie River upstream from the study site is approximately 400 square miles (1,035 square kilometers). The flood-plain surface is approximately 1 mile wide (16 kilometers) and the relief on the surface is about 4 feet (1.2 meters). Surface-water stage data from two stations upstream and one downstream from the study site were analyzed to determine the depth of the water and duration of flooding at the vegatation plots.

The surficial aquifer at the study site is about 30 feet (9 meters) thick and consists of alluvial sand and clay depostied by the Coosawhatchie River and older Pleistocene sand and clay. These deposits are underlain by a 40-foot (12 meter) thick confining unit, which in turn overlies the Floridan aquifer. Piezometes were installed in the surficial aquifer adjacent to and in the river channel at 22 locations to monitor the elevation of the water table at vegetation plots and determine the recharge-discharge relation between groundwater and surface water. Regressions of water-table elevations with the nearby river stage and four continuous recording piezometers on the floodplain at 18 piezometers yielded correlation coefficients that ranged from 0.02 to 0.99. Water levels in piezometers closest to the river generally correlated best with river stage. Initial measurements in the winter show that groundwater levels measured at a depth of several feet below the river bottom were lower than the river stage.

Historic decreases in ground- and surface-water flow may affect vegetation community dynamics, which possibly resulted in long-term shifts from wetter to dryer species compositions. The Floridan aquifer is about 70 feet (21 meters) deep and currently has a potentiometric surface approximately 1 foot (0.3 meters) above land surface at the study site. The potentiometric surface of the Floridan aquifer has declined approxmiately 15 feet (4.6 meters) at the study site during the last century due to groundwater withdrawals for municipal and industrial water suppies. Because upward leakage from the Floridan aquifer to the overlying surficial aquifer may contribute to the flow in the river during periods of groundwater discharge, long-term stream gaging records are being investigated to determine if the lowering of the potentiometric surface in the Floridan aquifer has resulted in long-term declines in surface-water flow.

--- May 1997

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