Date of Award

1990

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Geography and Anthropology

First Advisor

Richard H. Kesel

Abstract

This study assesses the geomorphological changes to the fluvial systems of two rivers in southwestern Mississippi that have been foreshortened. Channel foreshortening in the lower reaches of these rivers is similar to a lowering of base level and causes a steepening of the channel gradient which leads to channel incision, bank erosion and increased sediment load. Analysis of historical and modern data and a comparison with three unmodified rivers in the same physiographic region, provide an estimate of the magnitude and rate that such disturbances are transmitted through the fluvial system. Degradation and channel instability of the Homochitto River and St. Catherine Creek occurred immediately after foreshortening and proceeded upstream by knickpoint migration. A relationship was found to exist between the rate of knickpoint migration and drainage basin area of the mainstem channels and tributaries. Variations in the rates of channel incision and widening were related to changes in the regional geology. The increased bankfull capacity of the foreshortened channels has reduced the frequency of overbank flooding. The size of point bars along the Homochitto River have nearly doubled due to the introduction of enormous volumes of sediment from channel widening. Short-term channel changes also occurred in response to scour and fill associated with individual flood events. From these data a five phase model was formulated to illustrate the evolutionary change to the channel system. The meandering phase represents the channel prior to foreshortening. The degradation and aggradation phases indicate the sequential adjustments of the channel after knickpoint formation. The longitudinal profile in the final phase has reached a quasi-equilibrium position, although is located at a lower elevation indicating some degree of nonrecovery. Additionally, the recovery time varied both spatially and temporally within the fluvial systems with the tributary channels lagging behind that of the mainstem.

Pages

159

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