Hydrodynamics and SedimentTransport in the Lowermost Mississippi River: Implications for the Development of Fluvial-Deltaic Stratigraphy in Large Lowland Rivers Basins

Jeffrey Nittrouer
[email protected]

Where rivers enter the coastal zone, non-uniform hydrodynamic flow conditions develop within the active channel. The section of river affected by non-uniform flow is referred to as the backwater segment, and for large rivers, backwater flow extends tens to hundreds of kilometers upstream of the outlet. Field-observation and modeling studies from the modern Mississippi River document the persistence of backwater hydrodynamics, and its influence on sediment mobility in the lower five-hundred kilometers of the river. Reach-average shear stress varies temporally in accordance with the hydrograph, affecting the timing, volume, and grain size of sediment in transport. A reduction in shear stress limits the movement of coarse-grain (bedload) sediment, and this portion of sediment does not reach the ocean outlet. Coarse sediment is instead sequestered in the river channel, producing channel bed aggradation over time. This aggradation 'grades' the channel bed, pushing the backwater transition downstream, and generating a coarse-grain sediment front that slowly migrates downstream. However, bed aggradation also elevates the active channel bed and increases the likelihood for avulsion, whereby the channel is abandoned in favor of a new path to the ocean outlet. Following an avulsion, the abandoned and relatively inactive channel slowly fills with fine sediment, incorporating the partial coarse-grain sediment fill into the long-term deltaic stratigraphy. Using theoretical models, sediment wave propagation and channel avulsion timescales are calculated and compared for a wide-range of fluvial-deltaic systems. This modeling research predicts the likelihood that coarse sediment reaches the fluvial-ocean interface for an assortment of river systems.

AAPG Search and Discovery Article #90167©2013 GCAGS and GCSSEPM 63^rd Annual Convention, New Orleans, Louisiana, October 6-8, 2013