The lowermost Mississippi River: A mixed bedrock-alluvial channel

Jeffrey A. Nittrouer, David Mohrig, Mead A. Allison, Aymeric Pierre B. Peyret

Research output: Contribution to journalArticlepeer-review

84 Scopus citations


In this study, the distribution of channel-bed sediment facies in the lowermost Mississippi River is analysed using multibeam data, complemented by sidescan sonar and compressed high-intensity radar pulse seismic data, as well as grab and core samples of bed material. The channel bed is composed of a discontinuous layer of alluvial sediment and a relict substratum that is exposed on the channel bed and sidewalls. The consolidated substratum is made up of latest Pleistocene and Early Holocene fluvio-deltaic deposits and is preferentially exposed in the deepest thalweg segments and on channel sidewalls in river bends. The exposed substratum commonly displays a suite of erosional features, including flutes that are quantitatively similar in form to those produced under known laboratory conditions. A total of five bed facies are mapped, three of which include modern alluvial deposits and two facies that are associated with the relict substratum. A radius of curvature analysis applied to the Mississippi River centreline demonstrates that the reach-scale distribution of channel-bed facies is related to river planform. From a broader perspective, the distribution of channel-bed facies is related to channel sinuosity - higher sinuosity promotes greater substratum exposure at the expense of alluvial sediment. For example, the ratio of alluvial cover to substratum is ca 1·5:1 for a 45km segment of the river that has a sinuosity of 1·76 and this ratio increases to ca 3:1 for a 120km segment of the river that has a sinuosity of 1·21. The exposed substratum is interpreted as bedrock and, given the relative coverage of alluvial sediment in the channel, the lowermost Mississippi River can be classified as a mixed bedrock-alluvial channel. The analyses demonstrate that a mixed bedrock-alluvial channel boundary can be associated with low-gradient and sand-bed rivers near their marine outlet.

Original languageEnglish
Pages (from-to)1914-1934
Number of pages21
Issue number7
StatePublished - Dec 2011


  • Bedrock-alluvial river
  • Channel-bed facies
  • Mississippi River
  • River morphology
  • Sediment transport


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