EFFECT OF FRESHWATER DIVERSIONS ON MISSISSIPPI DELTA MARSHES BETWEEN 1984 AND 2009

Freshwater diversions, originally proposed to control flooding and later promoted to reduce salt water intrusion into fresh and intermediate Louisiana coastal marshes, have been a major marsh restoration strategy since the early 1990s. In January 2012, Louisiana State officials announced a fifty-year coastal restoration and protection master plan that included $4 billion to divert freshwater and sediment from the Mississippi River to stabilize and build new coastal marshes.

Our analysis of twelve Landsat data sets collected between 1984 and 2009, indicates that after two decades of operation, the Caernarvon, Naomi and West Pointe a La Hache freshwater diversions failed to substantially increase marsh vegetation health or marsh area. After Hurricanes Katrina and Rita in 2005, the marshes closest to the diversion inlets in Naomi and West Point a La Hache, and especially the Caernarvon diversion, showed more extensive vegetation and land loss compared to the less severe and transitory effects in adjacent reference sites.

We suggest that the susceptibility of marshes adjacent to diversion inlets to greater hurricane damage was due to high nutrient flux and the constant overwatering of marsh vegetation. Nitrogen and phosphorous concentrations in freshwater diversions increase aboveground biomass at the expense of rhizome and root growth, reduce soil strength and organic soil accretion rates and increase lodging or collapse of aboveground vegetation. Our results are consistent with other scientists who found that the introduction of Mississippi River water to coastal marshes creates a more reduced soil, increases decomposition of root systems, and produces a weakened soil that increases marsh vulnerability to hurricane damage.