Spatial Trends in Sediment Sorting and Stratigraphic Architecture within Lowland Rivers

Abstract:

A spatial reduction in bed material flux is observed where rivers transition from normal flow to the back-water influenced zone, as the river flow begins to feel the effect of sea-level in the receiving basin. The back-water length is a characteristic length scale of all rivers entering a receiving basin, but is best distinguished in lowland rivers with shallow gradient. We will quantify trends in stratigraphic architecture and sediment partitioning within the deposits of the Holocene Mississippi Channel Belt as a consequence of this change in river hydraulics at the back-water transition.

Measurements from Cairo to Head of Passes show that the back-water transition, in the vicinity of river kilometer 400, marks a dramatic reduction in the width of the Mississippi Channel Belt from 5 to 15 times the channel width upstream of the transition zone to nearly equal to the channel width downstream of the transition zone. This variation in channel belt width is tied to the decreased lateral mobility of the channel downstream of the back-water transition. The thickness of bank-attached bar deposits, collected from >1700 cores in 110 cross-sections, was used as a proxy for channel depth from Cairo to Head of Passes. Thickness trends reveal that bank-attached bars thicken from approximately 20m upstream of the transition to 45m just above Head of Passes, while decreased lateral migration result in narrower bar deposits. A comparison of 5 different channel belts from the Rhine-Meuse and Mississippi systems is presented. The data shows that when the channel belt width scaled by channel width is plotted against distance upstream scaled by the backwater length, a tight linear relationship emerges. This relationship is helpful when interpreting paleo-environment of deposition from ancient remotely-sensed channel belts in data with limited areal coverage.