Downstream change in the patterns of sediment deposition and erosion in the lower Mississippi River associated with varying water discharge

The downstream spatial deceleration of water flow in a river approaching its outlet defines the backwater condition. In the lower Mississippi River, backwater conditions occur over the lower 700 river kilometers and results from recent studies have shown that this condition has an important role for regulating the downstream transport of sediment. This study defines how the spatial variability in flow velocity and sediment transport varies as a function of river discharge. To perform these calculations we use 2650 measured cross sections of the channel, spaced roughly 300 m apart in the downstream direction. A characteristic flow velocity at each cross section is calculated by dividing the water discharge by its cross-sectional flow area. Local flow velocity is converted to total bed stress using an appropriate dimensionless drag coefficient. This drag coefficient is determined by optimizing the fit between a calculated and measured water-surface profile for eight discharges using a one-dimensional backwater model. Estimates for skin-friction shear stress are then produced from the values for total shear stress based on field data that constrain the fraction of total stress associated with form drag. Skin-friction shear stress values are used to calculate bed-material transport in the lower 800 kilometers of the river for a range of water discharges. These calculations demonstrate that during low- and moderate-water discharge, cross-sectional flow area increases downstream through the backwater segment, resulting in a decrease in water-flow velocity and bed-material transport. During high-water discharge, the trend is reversed and cross-sectional flow area decreases in the downstream direction, resulting in an increase in water-flow velocity and bed-material transport. To our knowledge this is the first documented reversal in cross-sectional area of a river channel associated with changing water discharge. Integrating bed-material flux over an annual hydrograph reveals a tendency of channel-bed aggradation in the upper reach of the backwater segment (150-600 kilometers above the outlet) and a region of channel-bed erosion in the lowermost 150 kilometers of the Mississippi River. How these results are connected to observed spatial variability of channel morphology and kinematics will be discussed in detail.