Submitted to: Federal Interagency Sedimentation Conference Proceedings
Publication Type: Proceedings
Publication Acceptance Date: December 15, 2000
Publication Date: N/A
Interpretive Summary: With nearly 80,000 substantial reservoirs constructed within the continental United States, concerns about reservoir water and sediment quality are increasingly common as many of these reservoirs reach the end of their life-expectancy and are considered for de-commissioning or over-haul. Over half of existing U.S. dams were constructed during the period from 1945 to 1975, and the average age of U.S. reservoirs is 40 years. This suggests the need for greater dam maintenance and/or major rehabilitation (National Performance of Dams Program [NPDP], 2000). NPDP (2000) also estimated that dam safety costs alone over the next 20 years could range from $750 million to $1.5 billion due to loss of capacity from sedimentation. Mitigation of environmental and potential human health impacts from reservoir sediments and waters could potentially result in exponential increases above that amount. More knowledge of the quantity and quality of sediments within reservoirs is needed to better address future management decisions. In this paper, we describe a study of Grenada Lake reservoir in north-central Mississippi, addressing questions of sediment quantity and quality within the lake, and the impacts on reservoir parameters from upstream watershed processes and modifications. Accumulating sediments are not presently creating significant water quality difficulties. They do not represent a threat to storage capacity. Natural Resource Conservation Service (NRCS) personnel and others interested in dam rehabilitation will be interested in this research.
Technical Abstract: Grenada Reservoir was commissioned in 1954 as part of a comprehensive plan for flood control in the Yazoo River Basin in northwestern Mississippi. The Yalobusha and Skuna rivers contribute inflow to the reservoir. Total watershed drainage area is approximately 3,419 square kilometers (1,320 square miles). Reservoir life expectancy was originally estimated at 25 years because of high erosion rates in the region. However, the lake continues to function with only slightly reduced storage capability. Reservoir and watershed sediments were sampled in 1998 and 1999. Sediment accumulation within the permanent pool adjacent to the dam was less than 1cm/yr except for a depositional area near tributary inflow that accumulated sediment at about 5 cm/yr. The central area of the permanent pool has experienced sediment accumulation rates averaging less than 1.5 cm/yr. 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 1960's & early 70's but were considerably lower thereafter. These lower sedimentation rates paralleled land use changes and followed discontinuance of major upstream channel alterations for flood control. 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 lake sediments and overlying water were generally low or not detectable. Conversely, several metals (arsenic, lead, copper, iron, aluminum and zinc) were abundant in stream and lake sediments.