New Orleans and the ecology of the Mississippi River

Richard Sparks writes about the ecological/geological context in which New Orleans exists, how people have changed them, and what rebuilders should consider. His article is Rethinking, Then Rebuilding New Orleans, in the Winter 2006 Issues in Science and Technology.

His article focusses on the natural forces that have shaped the Mississippi and how humans have shaped those forces. One of the most interesting points he raises is how land cover change, and river management have radically changed the sediment load of the Mississippi, shifting the balance between land building and subsidence in the delta. In other words flood protection higher in the river has made lower portions of the river more vulnerable to flooding.

sediment loads carried by the Mississippi River 1700 & 1980-1990Figure: The sediment loads carried by the Mississippi River to the Gulf of Mexico have decreased by half since 1700, so less sediment is available to build up the Delta and counteract subsidence and sea level rise. The greatest decrease occurred after 1950, when large reservoirs constructed trapped most of the sediment entering them. Part of the water and sediment from the Mississippi River below Vicksburg is now diverted through the Corps of Engineers’ Old River Outflow Channel and the Atchafalaya River. Without the controlling works, the Mississippi would have shifted most of its water and sediment from its present course to the Atchafalaya, as part of the natural delta switching process. The widths of the rivers in the diagram are proportional to the estimated (1700) or measured (1980–1990) suspended sediment loads (in millions of metric tons per year).

…It is ironic that the reservoirs on the Missouri, whose purposes include flood storage to protect downstream areas, entrap the sediments needed to maintain the Delta above sea level and flood level. Much less fine sediment (silt and clay) flows downstream to build up the Delta during seasonal floods, and much of this sediment is confined between human-made levees all the way to the Gulf, where it spills into deep water. Coarser sediment (sand) trapped in upstream reservoirs or dropped into deep water likewise cannot carry out its usual ecological role of contributing to the maintenance of the islands and beaches along the Gulf, and beaches can gradually erode away because the supply of sand no longer equals the loss to along-shore currents and to deeper water.

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