Can We Save the Mississippi?

The coastal United States has faced increasing threats due to climate change, with sea level rise, intense storms, and flooding battering the coastline. Louisiana is on the frontline of these threats. Since the 1930s, the state has lost almost 2,000 square miles of land. Some areas on the state’s coast are already grappling with the question of continuing to restore the land or retreat to avoid greater risks. Now, the impacts of sea level rise have reached freshwater systems as a mass of saltwater from the Gulf of Mexico travels up the Mississippi River.

The Mississippi River is the largest river in the country by volume and has the largest drainage basin, which covers 41% of the contiguous U.S. and extends slightly into Canada. The river delivers about 90% of freshwater inputs to the Gulf of Mexico and deposits sediment in the Mississippi River Delta. The Mississippi also provides freshwater supply for about 15 million people, serving as both a rich ecosystem and important resource throughout the Midwest and southern U.S.

The migration of saltwater up the Mississippi is caused by saltwater intrusion, a phenomenon in which saltwater permeates into freshwater environments, including surface water and groundwater. Saltwater intrusion in groundwater is well documented in several coastal states, including Florida, Georgia, and the Carolinas. This has led to various ecological issues, perhaps most notably ghost forests, or stands of dead trees due to high salinity levels.

In surface waters like rivers, higher density saltwater pushes under the freshwater layer. In the case of the Mississippi, decreased rainfall over the watershed has led to abnormally low water levels, meaning the river does not have enough force to push the saltwater back, especially as sea levels rise steadily.

While the Mississippi saltwater wedge has gained attention recently, this phenomenon is not new. Though infrequent, at least six saltwater intrusion events have been recorded between 1936 and 2023. In 1936, a saltwater wedge migrated about 114 miles upstream, while in 1988, the tip of the wedge (also called the toe) traveled over 104 miles up the river, making it to downtown New Orleans before subsiding. While the salt water was too deep in the river to have any noticeable effect on the city’s drinking water, barges of freshwater were needed to dilute the river in Plaquemines Parish to the southeast.

Today, water quality monitoring and historic data allow us to track the estimated location of the salt wedge toe. The wedge progressed more slowly than scientists originally anticipated. As of January 22nd, the U.S. Army Corps of Engineers estimated that the wedge toe now sits at river mile 11, where it no longer threatens the drinking water supply. The wedge began receding last October without reaching New Orleans due to higher rainfall. However, the impact on communities along the river is significant and lasting. Thousands of residents in Plaquemines Parish, a county in southeastern Louisiana, spent weeks drinking bottled water because salt concentrations in the tap water were too high. Residents even reported being able to taste the salt.

While the saltwater wedge has largely retreated for now, this threat will become more common as droughts and sea level rise intensify. The 2023 saltwater wedge was the second in two years, indicating that saltwater intrusion up the Mississippi River will likely become a more frequent phenomenon than in the past.

One solution the region has invested in is an underwater sill. The Army Corps of Engineers built the structure in response to the 1988 salt wedge. Because the denser saltwater sinks to the bottom of the river, the sill blocks its movement upstream until it builds up and flows over. The structure was augmented to be taller to slow the 2023 wedge. While the sill has proven effective in delaying the movement of the saltwater wedge, cities downstream of the sill are more vulnerable.

Coastal impacts of climate change not only threaten coastal land and infrastructure, but also our access to freshwater, demonstrating that the health of the ocean is essential to our own. In The Sea Around Us, Rachel Carson pointed out that the ocean “though changed in a sinister way, will continue to exist; the threat is rather to life itself.” Human-driven climate change has led to rising sea levels, which along with increased droughts now severely threaten communities surrounding the Mississippi River in Louisiana. Caring for the ocean and our freshwater resources will require combating the root causes of climate change.

Emma Brentjens – RCC National Environment Leadership Fellow (Presidential)

Emma Brentjens is the co-lead of the RCC Coasts and Oceans program. She is a Master of Environmental Management student at Duke University studying Ecosystem Science and Conservation and Community-based Environmental Management.