Linking the planform shape of channel deposits to spatial changes in channel dynamics across the backwater transition zone

Background/Question/Methods

Three-dimensional seismic data is often used to visualize the interior of thick sediment accumulations. Channel belts, the composite deposits left behind by laterally mobile channels, are easy to identify in seismically-imaged deltaic and fluvial environments. A few key questions inevitably arise while mapping channel belts in seismic reflections that represent “snap-shots” of past landscapes. Where was the shoreline? Can the planform shapes of channel belts help us invert environmental factors such as steady, rising or falling sea-level? We will discuss how the back-water length scale in rivers defines systematic changes in the widths and thicknesses of channel belt deposits across the transition from normal flow to the back-water influenced zone. A spatial reduction in bed material flux is observed where rivers transition from normal flow to the back-water influenced zone, where the river responds to the standing body of water in the ocean. The back-water zone is present in all rivers entering a receiving basin, but is pronounced in rivers with shallow gradient.  

Results/Conclusions

Measurements along the Holocene Mississippi Channel Belt from Cairo to Head of Passes show a dramatic reduction in the width of the channel belt from roughly 20 times the channel width upstream of the transition zone to nearly equal to the channel width downstream of the transition zone. This variation in width of the channel belt 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 USACE 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 less extensive bar deposits. A comparison is presented of the channel belts from the Rhine-Meuse and Mississippi river systems which have very disparate scales. For these two systems, we show how the backwater length and the average channel width can be used to non-dimensionalize the distance upstream of the river mouth and the channel belt widths respectively. The dimensionless shapes of these two systems collapse onto a single linear trend. Results suggest that the backwater length is a characteristic length scale in rivers, and defines systematic changes in channel dynamics and deposit geometry. This approach can therefore be used to reconstruct environment of deposition in ancient seismically-imaged channel belts.