Sedimentation in three small erosion control reservoirs in northern Mississippi

The water storage capacity and dam integrity of thousands of flood control reservoirs built since 1950 may be compromised by excessive stored sediments. The fate of these structures depends on tile amount and characteristics of accumulated material. To aid in understanding the scope of impairment of small reservoirs in the hill lands of northern Mississippi, physical sediment characteristics and reservoir storage capacity were evaluated in three small reservoirs (<8 ha <20 acres) built in the early 1960s as part of the Yazoo-Little Tallahatchie erosion control project. A vibrating corer was used to collect continuous cores of deposited sediment and preimpoundmnent soil. Bulk density, grain size, and activity of 137 Cs were used to identity the boundary between preimpoundment soil and postimpoundment sediment. Mean local sediment accumulation rates ranged from 5.6 to 7.9 mm yr -1 (0.22 to 0.31 in yr -1) with reductions in storage capacity of 7% to 18% since construction. Selected cores were tested for pre-1972 residual pesticides and more recently used pesticides. Results suggest that some currently used pesticides are being transported by prevailing winds from the heavily farmed Mississippi Delta area.