THE RESPONSE OF THE LOWER MISSISSIPPI RIVER TO RIVER ENGINEERING

An examination of the response of the Lower Mississippi River (LMR) to a variety of engineering activities is presented through the discussi on of: (a) a brief history of engineering investigations and activitie s on the LMR; (b) the impact of artificial cutoffs on the channel geom etry and water surface profiles of adjacent reaches; (c) the impact of channel alignment activities on channel morphology; and (d) the appar ent impact of all of the LMR engineering activities on sediment dynami cs in the channel. Investigations by many agencies reflect over 150 ye ars of study of the hydraulics and hydrology of the LMR, which have co ntributed significantly to our understanding of large alluvial rivers. In an effort to provide for hood control and navigation on the larges t river in North America, private landowners and the US Army Corps of Engineers have performed a wide range of river engineering activities, including construction of levees, floodways, artificial cutoffs, bank revetment, training dikes, dredging, channel alignment, and reservoir s on the major tributaries. This unprecedented program of river engine ering activities on the river during the last 100 years has resulted i n the evolution of a freely meandering alluvial river to a highly trai ned and confined meandering channel. The LMR has increased its overall gradient and average top-bank width and generally increased its chann el depth. The immediate response of the river to increased gradient as a result of the construction of artificial cutoffs was dampened in so me locations by local geological controls. Examination of the trends i n sediment dynamics of the LMR reveals that the suspended load of the river has decreased during the 20th century. Conversely, a trend in th e bed load transport in the channel for the years 1930 and 1989 cannot be determined with confidence because of the difficulty in acquiring representative samples. The highly trained river now responds to chann el forming flows by attempting to build mid-channel bars rather than n atural cutoffs of meanders. The LMR should maintain a relatively stabl e plan form in the intermediate future, barring a very large and unpre cedented flood. The river will continue to adjust its channel geometry and its local gradients as a response to variations in significant di scharges. Continued channel maintenance and occasional dredging will i nsure the present state of sediment and water transport efficiency.