How do you get sediment to accumulate until it reaches the water surface and becomes an island? Answering this question is essential in the quest to build new land and marshes on the Mississippi Delta, where vast tracts of land have sunk beneath sea level in the past century. Experiments performed at the University of Arkansas and the University of Wyoming provide one recipe.
The Mississippi River carries hundreds of millions of tons of sediment to the Gulf of Mexico every year. One portion of this sediment settles on marshes, allowing them to keep pace with rising sea level. One portion is flushed out to the continental shelf and beyond. A third portion accumulates in bays downstream of river channels and builds into a delta with islands and channels branching around them (as in the movie below). These are the few places where land and marsh are emerging where they previously didn’t exist. The Wax Lake Delta, one site of island formation, has produced about 12,000 acres of island and marsh in the past 45 years that is a prime duck hunting destination in coastal Louisiana (look at these happy sportspeople). The state of Louisiana has allocated $50 Billion to continue restoring the Mississippi Delta, and land building projects are central to the projects masterplan.
While islands often form where sand accumulates downstream of river channels, the processes that set the site of island formation and that cause sediment to build to the water surface are not well understood. Consider these two photographs below, taken about 3 miles apart on the Wax Lake Delta. Photo (b) is the shallow delta platform beyond the establishment of islands. Photo (c) is an established island. The transition between environment (b) and environment (c) below is the focus of this study.
The research team built a set of experimental deltas one 10-thousandth the size of the Wax Lake Delta to investigate what controls the location of island formation. The incoming water and sediment discharges and the angle that flow could spread into the basin were altered between runs. Experiments run previously in other research labs were also used as part of the Sediment Experimentalist Network, a new initiative funded by the National Science Foundation to increase data sharing between research groups to insure that findings are repeatable.
The team found that during the early stages of sediment accumulation, flow and deposition patterns changed rapidly. They started as a focused jet and sandbar directly downstream of the channel, but evolved into a circular bar that didn’t favor any flow direction. This circular deposit initially grew with flow and sediment transport over the entire the deposit (i.e. without islands), but after a certain amount of growth, islands began to form at relatively constant distance around the circular deposit. The blue dots and green arcs on the image below show the location of the islands on the experimental deltas.
By carefully observing these experiments (staring at them for hundreds of hours), the team found that islands began to form where the flow had enough energy due to spreading that it could no longer transport the sand that was supplied from upstream. This caused islands to form with focused channels between them that allowed sand transport to persist further downstream. This distance formed a circle around the center of flow spreading. The team built a simple model predicting that island location would be controlled by water discharge, flow depth, grain size and the angle of spreading. Double the water input and the islands should double their distance. Double the spreading angle, and the island distance cuts in half. The model prediction (red lines) could predict island locations that varied by a factor of 10 with an error of less than a factor of two. Furthermore, the model predicted the location of islands on the Wax Lake Delta and other field sites with the same accuracy. A paper publishing these findings was just published at the Journal of Geophysical Research — Earth Surface.
This study provided an answer to two questions. What controls island formation? It occurs at locations where flow is moving too slowly to transport sediment from upstream. Where does it occur? Sediment deposition patterns and flow evolve together until flow expands radially, and the location of island formation can be predicted as a radial distance downstream of this spreading center. One of the primary goals of Mississippi Delta restoration efforts is to build more land and coastal marsh. The findings published here provide new insight to the recipe for island formation.
John Shaw (@johnburnhamshaw) is an Assistant Professor at the University of Arkansas and runs a research program studying erosion and deposition on river deltas. Kimberly Miller (@klitwin) is a post-doc at the University of Wyoming and is an expert on sediment transport in many settings. Brandon McElroy is an Assistant Professor at University of Wyoming and studies sediment transport on rivers and deltas. The study was run by these three scientists, as well as many undergraduate students from University of Arkansas and University of Wyoming. This study was funded by the National Science Foundation.