Sedimentologische und geomorphologische Prozesse des Unteren Mississippi

The Lower Mississippi River, a several thousand years old graded meandering river system, became strongly affected in its equilibrium by a river regulation program. This included cutoffs (1929–1942), crossectional changes by dike systems, and revetments. The cutoff program changed the river in such a way that sinuosity and slope shifted into the reach where the river starts to develop a braided channel. This is indicated by the increasing number of sandbars with time. Dike constructions decreased the river width in the pool areas causing incision there but consequently also widening and shallowing of the river crossings. Here the river is shallower and wider than ever before. The increase of the width/depth ratio of the river crossings is correlated to decreasing sinuosity. The alteration of the river's regime is probably best documented by the changes of the rating curves with time. The cutoff program initially lowered the stages for given discharges, but since 1950 a reverse trend, particularly for flood flows, can be observed. This indicates a decrease of the mean crossectional area and/or a raising of the mean stream bed elevation. The 1973 flood and a particular reach of the river are regarded as examples in this context. From the engineering point of view the present Lower Mississippi River is less efficient than it was before the river regulations. From the sedimentological and geomorphological point of view the recent processes can be regarded as the result of a large scale experiment, modelling a river's adjustment to changed conditions.