A Mississippi flood control reservoir: life expectancy and contamination

The fate of reservoirs is a major water management, water quality, and aquatic life use concern across the globe. We examined sedimentation rates, current watershed contamination contributions and potential impacts of long-term row cropping (cotton, corn, soybeans, and sweet potato) in a large flood control reservoir in the loess hills of Mississippi, USA. Grenada Reservoir, created in January 1954, was constructed as part of a comprehensive plan for flood control in the Yazoo River Basin in northwestern Mississippi. Two rivers, the Yalobusha and Skuna, contribute inflow to the reservoir, forming a distinctive Y-shape within the topography of the flood pool. Total watershed drainage area for the reservoir is approximately 3,419 square kilometers (1,320 square miles). Reservoir sediment accumulation rates were sampled in 1998 and 1999. Although reservoir life expectancy was originally estimated at 25 years because of high erosion rates in the watershed, our study revealed that the reservoir continues to function with only slightly reduced storage capacity. Sediment accumulation within the permanent pool adjacent to the dam was < 1cm yr.1 except for a depositional area near tributary inflow that accumulated sediment at about 5 cm yr.1. The central area of the permanent pool experienced sediment accumulation rates that averaged < 1.5 cm yr.1. Sites within the two reservoir arms fed by the two river inflows showed little or no sedimentation. Sedimentation rates further upstream in these two inflow areas were also generally low. Sedimentation rates within Grenada Reservoir were higher until the mid 1960s & early 70s but were considerably lower thereafter. These lower sedimentation rates paralleled land use changes and followed discontinuance of major upstream channel alterations for flood control. From 1996 to 2002 analyses were conducted in water and sediment for 8 metals and 48 pesticides/ contaminants at 26 stream/river locations and 9 locations within the reservoir. In spite of long-term historical use of residual pesticides in the watershed and widespread use of currently applied agricultural compounds, concentrations in stream or reservoir sediments and overlying water were generally low and sporadic or not detectable. Conversely, several metals (arsenic, lead, copper, iron, aluminum and zinc) were abundant in stream and reservoir sediments. Atrazine, a triazine herbicide, was routinely found in stream water and sediment. It was also detected in reservoir water samples but at nearly one fifth less than contributing stream concentrations. Naturally occurring aluminum and iron were found in high concentrations. Residual pesticides were generally not detected in water but were detected in stream and reservoir sediments.