Environmental Assessment, Water Control Structure Improvements in Mississippi River Pools 20 and 21, River Miles 347 to 350 and 332 to 333, Lewis Country, Missouri and Hancock County, Illinois.

摘要

The U.S. Army Corps of Engineers, Rock Island District, is responsible for maintaining the Mississippi River Nine-Foot Channel Navigation Project in Pools 11 to 22. A navigation channel depth of 9 feet minimum must be maintained from early spring to winter for commercial towboat traffic. The 9-foot depth is basically maintained by the series of locks and dams along the river. The river, however, is still in a semi-natural condition in terms of sediment (bedload) transportation. At times, this sediment accumulates at various locations in the navigation channel and must be removed by dredging in order to prevent channel closure and stoppage of commercial traffic. Sometimes this shoaling (sediment accretion) happens so rapidly that emergency dredging is required to reopen the channel where towboats run aground. The purpose of these structures is to direct the flow of water away from shorelines, side channels, etc., and into the main channel. Basically, these structures constrict the river flow into a more narrow cross section. This reduced cross section causes the water velocity to increase and subsequently to transport more sediment. (sdw)