By Alisha A. Renfro, Ph.D., Staff Scientist, National Wildlife Federation
Last week, an independent scientific panel comprised of prominent scientists from throughout the U.S. released a report, “Mississippi River Freshwater Diversions in Southern Louisiana: Effects of Wetland Vegetation, Soils, and Elevation,” which examines some of the ecological effects of freshwater river diversions. The panel concluded that there is little evidence suggesting that the existing freshwater diversions in Louisiana have appreciably reversed the rate of land loss in the region, and that to reverse the land loss trend, significant inputs of sediment are needed. While most of the existing diversions in Louisiana were built to move fresh water only, many of the diversions included in Louisiana’s 2012 Coastal Master Plan focus on sediment capture and conveyance into coastal wetlands.
Freshwater diversions affect basins by reducing salinities. Extensive dredging of canals throughout the Mississippi River Delta’s wetlands has allowed for salt water from the gulf to intrude into wetlands adapted to lower salinity conditions, resulting in large areas of these wetlands dying and being converted to open water. Wetland vegetation is affected directly by the salinity of the water in wetland soil. High salt concentrations in the soil can affect vegetation by reducing the overall rate of photosynthesis, decreasing nutrient uptake and stunting growth rates. Consequently, the introduction of fresh water into wetland communities damaged by saltwater intrusion is vital in any restoration effort.
Freshwater diversions also increase the amount of nutrients introduced into the receiving basin. While increases in nutrient availability to wetland vegetation would presumably stimulate growth, scientific information collected in Louisiana marsh communities have exhibited varying results depending on plant species, nutrient concentrations and the abundance of different types of nutrients. Increasing the amount of nutrients may also alter the composition of the plant community, as some species of plants have a competitive advantage when it comes to nutrient uptake and growth.
River diversions can also have an influence on wetland elevation. In order for wetlands to persist over time, processes that increase the surface elevation of the wetlands must be equal to factors that increase the threat of submergence (e.g. sea level rise, storms). Diversions have the potential to promote an increase in the elevation of a wetland by adding mineral sediment to the surface and stimulating plant growth both above and below ground. However, the surface elevation of a wetland could decrease as nutrients become less scarce, as the abundance of vegetation roots decline and as an increase in the breakdown of belowground organic material by bacteria takes place. More scientific studies are needed to enhance our understanding of the relationship between marsh response and river input in order to better predict the net effect that sediment and freshwater diversions may have on different marsh types.
This scientific panel found that any freshwater diversion that does not transport a substantial sediment load is unlikely to reverse the current trend of wetland loss in Louisiana. The 2012 Coastal Master Plan recognizes and addresses this reality by focusing on large-scale diversions that would be capable of transporting significant amounts of river sediment into the nearby wetlands. In addition to shifting the focus of diversions from fresh water to sediment, the panel determined that a formal adaptive management scheme is needed for existing and planned diversions where the goals of the project are clear, the pre-diversion conditions of the affected area are well characterized, monitoring in the outfall area is done to measure the progress of the project in relation to its goals and a process exists to adjust the operation of the structure to increase the likelihood those goals are reached.
- Fact sheet: "Pulsed" land-building sediment diversions
- Mississippi River Freshwater Diversions in Southern Louisiana: Effects of Wetland Vegetation, Soils, and Elevation (Technical Panel from the Workshop on Response of Louisiana Marsh Soils and Vegetation to Diversions)