Pool-Riffle Morphology in an Actively Migrating Alluvial Channel: The Lower Mississippi River

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This study analyzes the pool-riffle morphology of the Lower Mississippi River within the context of an actively migrating meandering river. Historic hydrographic surveys (1:20,000) from the late 1800s and early 1900s document the natural channel morphology of the Lower Mississippi, and are used to construct a 1225-km profile of the channel thalweg. Previous studies suggest that pools and riffles are limited to gravel bed channels. This study documents their occurrence within a large fine-grained channel, and illustrates the importance of these features within the context of meandering processes. The channel width at riffles is significantly greater than at pools, and the spacing of pools is scale dependent, averaging 7.2 channel widths. Extreme values occur where heterogeneous channel deposits control the channel alignment. Meander-bend radius of curvature and pool depth is inversely related, which would occur because of the enhancement of helical flow and channel bed shear stress associated with sharper meander-bend curvature. The migration of meander bends increases with pool depth. However, the adjustment of pool depth is inversely related to meander-bend migration. Higher rates of lateral migration are associated with a reduction in pool depth, while meander bends that were less active laterally were more effective at scouring the channel and thereby increasing the depth of pools. The results of this investigation of a large and complex alluvial channel provides linkages with detailed studies that document the mechanisms of meander-bend processes within individual channel reaches and laboratory flumes. Findings suggest that the pool-riffle adjustment of meander bends occurs synchronously with planform adjustment, and as suggested by Keller and Melhorn (1978), represents meandering in the vertical dimension.