Recurrent shoaling and channel dredging, Middle and Upper Mississippi River, USA

A 34-year record of dredging in a 484-km reach of the Middle and Upper Mississippi River documents the spatial and temporal patterns of bed aggradation in an intensively engineered river. Between 1964 and 1997, 183 million m3 was removed from the study reach, with 112.6 km of the channel undergoing dredging of some kind, 12.1 km requiring ≥5 dredgings during this period, 2.6 km requiring ≥10 dredgings, and one site requiring 29 dredgings in 34 years. Forty-three sites were identified where dredging volume was exceptionally high and/or were frequently repeated. These sites occur in five settings: (1) where flow is divided through chutes or side channels; (2) at the mouths of largely unregulated tributary streams; (3) at thalweg crossings in meander bends; (4) in long, straight reaches of the channel; and (5) near problematic engineered structures, the principal one in the study area being the outlet of the Chain of Rocks Canal. In the pooled reach of the Upper Mississippi, impoundment has had a major influence on shoaling, with 80% of dredged volume required in the upstream halves of the pools. Although past engineering modifications have reduced sediment aggradation in the Mississippi channel, structural solutions to continuing shoaling can adversely impact the riverʹs ecology, flood response, or hydrological function, and many structural solutions appear to be approaching their effective limits. The exception to this may be at highly localized shoaling locations, such as the Chain of Rocks Canal outlet, where flaws in the original design may still be remedied by structural means Targeted structural solutions may be effective at solving limited local shoaling problems, but we suggest that more sweeping new engineering modifications to reduce or eliminate dredging requirements are likely to be ineffective and/or outweighed by their adverse effects on the river system.