Effects of Pool Drawdown and Wing Dams (Pool 8), and Closure Damns (Pool 13), on Navigation Channel Sedimentation Processes, Upper Mississippi River

Abstract : Construction of navigation locks and dams on the upper Mississippi River about 60 years ago submerged wing dam training structures, thereby reducing their effectiveness and increasing secondary channel and floodplain conveyance. The U.S. Army Engineer District, St. Paul, executed a drawdown of Pool 8 (upstream of Lock and Dam No. 8) near La Crosse, WI, during the summers of 2001 and 2002. Water levels were allowed to drop below normal minimum values to expose mud flats, promote seed germination, and benefit fish and wildlife. By lowering water levels during a drawdown, wing dam training structures submergence and floodplain conveyance will be decreased, and flow patterns around the training structures will be altered. This could result in sediment mobilization and scour in the navigation channel. During the spring of 2001, three closure dams were constructed in Pool 13 (upstream of Lock and Dam No.13) by the U.S. Army Engineer District, Rock Island, near Savannah, IL. These closure dams are actually submerged weirs that should allow water to continue to flow into the backwater areas of the islands of Pool 13, but at reduced rates. At issue is whether the main channel might require reduced dredging in future years as a result of the construction of the closure dams and, also, whether the backwaters of the eastern side of the islands will fill with sediment.