Meandering Processes and Channel Morphology in the Lower Mississippi River Prior to Major Human Disturbance.

Abstract

Although the Lower Mississippi River has been intensively studied, few studies have investigated meandering processes in the river when the channel was considered to be relatively free of major human disturbance. This research is significant in that it provides a detailed examination of meandering processes and channel adjustment in a large fine-grained alluvial setting, offering a departure from the base of knowledge on this topic that has evolved from studies on smaller coarse-grained river systems. The study spans the entire length of the Lower Mississippi River (1,704 km), from Cairo, IL to Head of Passes, LA. Two sets of hydrographic surveys (scale, 1:20,000) completed between 1877 and 1922 provide the primary source of data. Each survey was segmented into individual meander bends and straight reaches, digitized, and entered into a GIS for analysis of channel processes. Several techniques were employed to examine spatial and temporal change, and interrelationships between channel parameters and controlling factors, including; simple and multiple linear regression analysis, Independent and Paired t-tests, and Pearson correlation coefficients. Two major meandering regimes are identified, which coincide with the boundaries of the alluvial valley and deltaic plain. Channel morphology in the alluvial valley is characterized by its planform morphology having a sinuous laterally migrating channel. Although the cohesive sediments in the deltaic plain reduce lateral migration, the river is able to scour the channel bed into a uniform pool and riffle morphology. Migration rates for channel segments in the alluvial valley and deltaic plain averaged 26.7 and 3.3 m/km/yr, respectively. Analysis of meander bend migration rates with the adjustment of channel curvature between the two surveys suggests an equilibrium meander bend curvature, r$\sb{\rm c}$/W, between 3.0 and 4.0. Spatial trends in sinuosity, channel width, and radius of curvature do not conform to common downstream patterns due to the lack of a downstream trend in discharge. The planform morphology in the alluvial valley increased in sinuosity and decreased in radius of curvature, as meander bends became increasingly arcuate. However, channel width and the pool and riffle morphology did not significantly change during the study period.