Mississippi River Diversions Workshop Tackles Difficult Scientific Questions

By Angelina Freeman (Environmental Defense Fund), David Muth (National Wildlife Federation), and Bryan Piazza (The Nature Conservancy)

The Louisiana Coastal Area Program (LCA) Science and Technology Office and the National Oceanic and Atmospheric Administration (NOAA) convened a meeting Feb. 23-24 on the technical issues of freshwater river diversions and the response of wetland soils and vegetation.  The plants in coastal wetlands will drown if they cannot keep up with rising water levels.

This fact is especially evident in Louisiana, where the gradual rise of sea level is made relatively worse by rapid rates of subsidence (sinking land).  To maintain surface elevations within the intertidal zone, wetlands need to add soil.  Luckily, both by capturing sand and clay from the water and – just as importantly – by adding organic matter through root growth and leaf drop, healthy wetlands can increase their elevation.

Photo by Dennis Demcheck (USGS)

One critical tool for ensuring the health of coastal Louisiana is to reconnect the wetlands to the river with diversions of river water and sediments.  Diversions mimic the natural delta cycle that was interrupted with river levees and channelization.  It is important that we develop a thorough understanding of the effects of river diversions on wetland plants and soils so that future management and restoration are based on a sound understanding of the biophysical controls on soil surface elevation and how they can be optimized for restoration.

Nutrient inputs to the Mississippi River and its tributaries have increased in recent decades, creating a desire for basin-wide nutrient mitigation strategies to alleviate problems, such as Gulf hypoxia (when oxygen concentrations fall below the level necessary to sustain most animal life, a.k.a. the Dead Zone).  While coastal wetlands have been shown to efficiently take up nutrients, there are questions regarding impacts of excess nutrients to wetland structure and function, which underscore the importance of nutrient reduction.

Scientists at last week’s workshop explored these questions from multiple perspectives and presented research investigating controls on marsh elevation, factors determining marsh soil and vegetation response,  marsh characterization, consequences of river diversions to belowground productivity, and effects of salinity and nutrients on plant growth and soil organic matter decomposition.  Abstracts of the presentations can be found on the U.S. Army Corps of Engineers’ website.

Freshwater diversions are useful for multiple purposes, including: protecting drinking water, providing nutrient inputs to maintain plant production, and maintaining down-basin salinities to increase fish and wildlife productivity.  Most of the diversions in Louisiana are called “freshwater diversions” because their discharge contains relatively little sediment.  Many of the speakers agreed that for the restoration of Louisiana’s wetlands, it is important to maximize the amount of sediment in diversions for land building.

A position paper will be developed by a technical panel to address what is known about Mississippi River diversions, where chief uncertainties are, the direction on science needed to reduce uncertainties, and recommendations for operation of existing structures.  Presentation abstracts, other workshop information, and post-workshop products can be accessed on the workshop website.