Evaluating potential sediment transport changes in lower Mississippi River due to engineered meander cutoffs in the 20th century: no case for large-scale flux increases

A variety of planform, hydraulic, and sediment transport changes in the lower Mississippi River have been ascribed to meander cutoffs between Memphis and Red River Landing (covering a distance of 700 km), occurring over the period 1929-1942. Some impacts are clearly demonstrated, including a reduction in stage for a given discharge, as is measured at numerous gaging stations. However, another phenomenon repeatedly called upon in literature, specifically an enhancement of bed material flux at the scale of 100s of kilometers, maintains little quantifiable support. The proposition that increased bed material flux in the lower Mississippi River is a result of the cutoffs has implications for interpreting measured aggradation near the downstream end of the cutoffs in the vicinity of the Old River Control Complex. Here we test the hypothesis that cutoffs in the lower Mississippi River raised longitudinal slopes, and concomitantly boundary stress, so as to increase bed material flux and ultimately downstream sediment delivery. We explore three main aspects of this hypothesis to assess its validity: 1) verifying alterations to hydraulic geometry, 2) leveraging these measurements to inform models of equilibrium transport, and 3) conserving sediment masses throughout the segment of cutoffs. Channel length was originally reduced by approximately 240 km, however natural meandering added back 80 km. The mean longitudinal slope increased from 5.7x10^-5 to 7.0x10^-5. As a result of reach-scale incision, bankline stability was compromised, and the channel widened. Following, numerous mid-channel bars were added, and hydraulic roughness increased. These planform adjustments lead to the placement of revetments and wing dykes, which stabilized the channel in its current position. Using post-cutoff boundary conditions to assess the instantaneous alteration of the sediment transport fields, we calculate that incision due to reach-scale slope enhancement produced augmented sediment flux at scales of 20 kilometers or less with downstream deposition re-establishing morphodynamic equilibrium. Mass conservation requires a much greater increase in sediment transport than has been observed or is required to build the accumulated sediment around Red River Landing.