Section 2.4: Impacts on the Ecology and Geology of the Gulf Coast
The construction of the system of levees and jetties in the Lower Mississippi Valley has had profound and perhaps irreversible consequences for the problem of coastal erosion in Southeast Louisiana—with the most important consequence likely being a fundamental alteration of sediment deposition patterns in the delta. Before the construction of the levee system, the Mississippi entered the Gulf of Mexico through a network of dispersed distributary channels, which interacted with freshwater wetlands and lakes in complex ways. Water that flowed into the alluvial fan of the Mississippi slowed down and dropped its sediment, which built up land surfaces and caused the coastline of the Mississippi Delta prograde. In this way, the Mississippi River was a crucial source of sediment that prevented coastal erosion, maintained freshwater wetland ecosystems, and created new land surfaces as those away from the Mississippi Delta eroded.
The current system of levees restricts the flow of water through distributary channels (except for the Atchafalaya River during extreme floods when the West Atchafalaya and Morganza flood- ways are opened). Furthermore, levees and jetties keep the water in the channel of the Mississippi flowing at a high velocity in order keep the channel clear of silt. This design prevents fine-grained sediment from accumulating in the Mississippi River delta and propels it far out into the Gulf of Mexico, negating most of the potential positive effects that this sediment might have in preventing coastal erosion and/or growing new land surfaces.
As discussed in the previous section, coastal land surfaces are either experiencing sedimentation from river systems or they are in a process of eroding. By eliminating natural processes of sedimentation in the Mississippi Delta, our system of levees and jetties has unintentionally accelerated processes of coastal erosion and land loss in Southeast Louisiana.
The construction of the levee system has had other consequences for the landforms and ecosystems of the Gulf Coast, as well. One of these consequences stems from the prevention of overbank flooding on the Mississippi River and its distributaries, which (prior to human intervention) occurred on an annual cycle. Overbank flooding provided water, fine-grained sediments, and organic material to the low-lying wetland areas in the Mississippi Delta.
While this approach was used to reclaim upstream portions of the Lower Mississippi Valley floodplains for farm land and settlements, many coastal wetlands lost their sources of freshwater, sediment, and organic nutrients. This has made coastal wetlands along the Mississippi Delta prone to saltwater intrusion, resulting in the death of plant communities that had been crucial in preventing erosion. In combination with the loss of sedimentation from the Mississippi, these ecological changes have had disastrous consequences in terms of the loss of land along the Gulf Coast of Southeast Louisiana.
Finally, the construction of the levee system in the Lower Mississippi Valley had one other major consequence for coastal land loss in terms of its effects on geological subsidence. As discussed in Part 1, clays and other fine-grained sediments shrink when they dry; the process called clay dewatering. In preventing overbank flooding from the Mississippi, as well as water flow through its delta distributaries, the levee system prevents water from reaching land surfaces composed of fine-grained sediment. This has resulted in the shrinking of sediments and the sinking of land surfaces in the Mississippi River delta. This process accounts for the high rates of subsidence in places like New Orleans, much of which was once covered by wetlands watered by overbank flooding and water flow through the distributaries of the Mississippi.