Abstract
Coastal wetland restoration can be complex and expensive, so knowing long‐term consequences makes it important to inform decisions about if, when, and where to conduct restoration. We determined temporal changes in land gain and loss in receiving basins and adjacent reference areas for two diversions of the Mississippi River in south Louisiana (Davis Pond and Caernarvon initiated in 1991 and 2002, respectively). Water from both diversions went into receiving basins with vegetated areas as did the adjoining reference areas. The results from two different types of satellite imagery analyses demonstrate a net land loss after diversions began. The results were confirmed for the Caernarvon diversion using a before–after/control–impact analysis of independently collected data over a larger area of the estuary. These results are consistent with an analysis of land gain and loss after a natural levee break on the Mississippi River in 1973. The positive influences of adding new sediments were apparently counter‐balanced by other factors, and consistent with the conclusion from other studies indicating that increased nutrient supply and flooding are, by themselves, negative influences on marsh health. Modeling the ecosystem effects of diversions can be calibrated and tested using landscape‐scale analyses like this to understand the chronic and delayed effects, including the unintended consequences. Basing the legitimacy of river diversion on ecosystem modeling will be premature without successfully reproducing empirical results like these in ecosystem models.
Highlights
The availability of dredges, pumps, conveyance pipes and channels, etc., makes it possible to move large amounts of sediments and water over great distances or to restrict water movements with an extensive system of levees
A large and modern restoration project on the Louisiana coast is where about 25% of the 1.95 million hectare coastal wetlands existing in the 1930s are open water (Couvillion et al 2016)
A third data set measuring land in the Caernarvon diversion area is from the Coastwide Reference Monitoring System (CRMS) administered by Louisiana’s Coastal Protection and Restoration Authority
Summary
The availability of dredges, pumps, conveyance pipes and channels, etc., makes it possible to move large amounts of sediments and water over great distances or to restrict water movements with an extensive system of levees. The changes in the West Bay diversion (birdfoot delta) that are used to validate their model are for an area that has open water areas overlying mineral soils, and the model uses sediment deposition, not land area, as the metric. The far-field inundation of wetlands, is not significant in the West Bay diversion, unlike for the two proposed diversions located northward and halfway to New Orleans (Brown et al 2019). These latter two diversions go into shallower water with wetlands. The results, regardless of whether there is a land loss or gain, can populate models with on-the-ground data, reduce the uncertainty in model predictions, and be used to inform policy
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