Potential effects of dredging the South Fork Obion River on ground-water levels near Sidonia, Weakley County, Tennessee

The U.S. Army Corps of Engineers has proposed dredging an approximate 7-mile reach of the South Fork Obion River near Sidonia, Tennessee. This dredging will have an effect on ground-water levels in a wetland area near the river. The river cuts into a sandy aquifer that is confined by an overlying clay layer. Ground water in the confined aquifer flows from a ridge on the north side of the study area toward the river, Estimates of aquifer transmissivity range from 3,300 to 18,800 feet squared per day. Assuming a 3-foot decline in stream stage due to dredging, the maximum decline in ground-water levels would be about 2.4 feet at low flow. Ground-water levels in the aquifer would decline by at least 2 feet at a distance of 0.5 mile from the river within 60 days after the change in stream stage, regardless of the assumed transmissivity value. Water-level declines in the upper clay layer probably would be much smaller. The time required for a specified change in ground-water level is dependent on the aquifer properties and distance from the river.