HR: 0800h
AN: H51I-0893    [Abstracts]
TI: Lower Mississippi River: A Mixed Bedrock-alluvial Channel
AU: * Nittrouer, J A
EM: nittrouer@mail.utexas.edu
AF: Department of Geological Sciences, University of Texas-Austin, Austin, TX 78712, United States
AU: Mohrig, D
EM: mohrig@mail.utexas.edu
AF: Department of Geological Sciences, University of Texas-Austin, Austin, TX 78712, United States
AU: Allison, M A
EM: mallison@mail.utexas.edu
AF: Institute of Geophysics, University of Texas-Austin, Austin, TX 78758, United States
AB: High-resolution multibeam bathymetric and CHIRP sub-bottom seismic data from the lower most 110 km of the Mississippi River offer a detailed depiction of the bed surface, revealing portions of the channel covered by sediment and areas of exposed substrate. A single line data swath running from Head of Passes to the English Turn bend was collected during low water discharge conditions. The swath zigzags up the river channel, intersecting each bank at an oblique angle of roughly 45 degrees, thereby ensuring that the entire channel bottom is adequately represented by the survey. Local composition of the channel bed is determined based on distinctive bathymetric characteristics, and is corroborated by grab samples collected along the survey path. Dune fields, composed of active and inactive dunes, cover 77% of the channel bottom and are most abundant in straight reach segments. Stagnant dunes, mantled by high-porosity ephemeral mud, cover 7% of the channel bed and tend to be located in shallower water near the river banks (10-15 m depth). Active bedforms cover 70% of the bed and are centered on the channel thalweg at water depths of 15-25 m. The remainder of the river bed is devoid of dunes, and consists of highly-consolidated and stratified sedimentary deposits that are part of the Pleistocene and Holocene substrata that lies beneath the modern Mississippi River (23% coverage). Steeply dipping channel sidewalls (30-80 degrees) are composed of the relict sediments and make up 10% of the bed area. Erosion-resistant substrate covers 13% of the channel bottom, and is frequently exposed in the deepest portions of the thalweg (25-60 m depth). Grooves and flutes are cut into this exposed substrate implying active river incision. Based on the proportion of modern sediment cover versus exposed erosion-resistant substrate on the channel bed, we propose that the lower Mississippi River is best classified as a mixed bedrock-alluvial channel.
DE: 1825 Geomorphology: fluvial (1625)
DE: 1862 Sediment transport (4558)
SC: Hydrology [H]
MN: 2007 Fall Meeting