Plaquemines Gazette Letter to the Editor: River Diversions
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Many people today are asking questions about what river diversions will do to our coast. These questions are particularly important in Plaquemines Parish, where several of the diversions in Louisiana’s coastal Master Plan will be located. Central to the Master Plan is the idea that sending Mississippi River and sediment into degrading wetlands will bring the materials needed to build land. But some are concerned. They note that areas near the Caernarvon Diversion lost land during Hurricane Katrina, and no one wants to see a repeat of that.
As a coastal geologist, the controversy of whether diversions lead to land growth or erosion intrigued me. My research team looked to Plaquemines Parish for an answer because the network of marshes on the bayside of Cubit’s Gap works like a large river diversion. This delta formed when oyster fisherwomen cut a channel in the Mississippi River levee, creating a, “natural,” diversion that built 75 square miles of land in under a century. Today, the system diverts large amount of river water, and my team used it to test whether diversions do or do not build stable land.
We used satellite images to determine how much land grew and eroded between 2004 and 2014, a period that included near-direct hits from Hurricanes Katrina and Isaac, glancing blows from Ike and Gustav, and several tropical storms. We went to these marshes and collected samples. We measured strength of the soil, its salinity, and the amount of roots and amount mineral sediments in the soil.
What we found was fascinating; some regions of this delta were growing while others were eroding. The wetlands closest to the Mississippi River were the ones building land. These river-influenced wetlands also had the strongest soils, the soils with the most mineral sediments, and the water here was freshwater. Land loss mostly occurred in marshes far from the Mississippi River. These were the marshes with the lowest soil strength, and the least amount mineral sediments in the soil. These eroding marshes lived in the highest salinity water.
The findings of this study were clear: the key to building stable land in a river diversion is to deliver lots of sediment to marshes.
Is Cubit’s Gap a good example of a large diversion, like the mid-Barataria diversion in the Master Plan? In my view, yes. Cubit’s Gap carries between 35,000 and 100,000 cubic feet per second of freshwater, similar to the 75,000 cubic feet per second that mid-Barataria will carry at high flow. In contrast, Caernarvon only carries about 1,000 cubic feet per second of freshwater. We realized the problem with Caernarvon was not the freshwater, it was the lack of sediment.
As a scientist and educator, I appreciate the questions that have been raised about diversions, because they force the scientific community to look critically at the science underlying our coastal knowledge, and to field test ideas about how nature works. After taking our test, large diversions passed.
Alexander S. Kolker, Ph.D.
Associate Professor Louisiana Universities Marine Consortium