An 80-year record of sediment quality in the lower Mississippi River

In 1937, the US Army Corps of Engineers cut through the "neck" of a large meander on the lower Mississippi River (below the confluence with the Ohio River) forming the Caulk Neck cutoff and creating Lake Whittington, a 26-km long oxbow lake, in northern Mississippi. Since 1938, seasonal flooding and a boat channel connecting the lake with the Mississippi River have led to sediment accumulation in the lake, resulting in an 80-year record of sediment quality in the river. On the basis of an age-dated sediment core from the lake, trends in trace metals and hydrophobic organic compounds (except polycyclic aromatic hydrocarbons) follow well-known patterns with upward trends from the 1930s to the ca 1970s, followed by downward trends to the present. Two factors contribute to these patterns: reservoir construction and changes in emissions. The construction of seven large reservoirs on the Missouri River, in particular the closure of the Fort Randall (1953) and Gavins Point (1955) Dams, greatly reduced the load of relatively clean sediment to the Mississippi River, likely contributing to downstream increases in contaminant concentrations in the Mississippi River. Increasing anthropogenic emissions also contributed to upward trends until ca 1970 when major environmental policy actions began resulting in broad decreases in emissions and downward trends in the concentrations of most of the contaminants monitored. Polycyclic aromatic hydrocarbons and phosphorus are partial exceptions to this pattern, with increases to the 1960s and variable concentrations showing no clear trend since. Published 2012. This article is a U.S. Government work and is in the public domain in the USA.