6.6 Mississippi Management
Consequences of Managing the Mississippi River System
Overview of River Management
In this segment, we explore the consequences of how we have managed the Mississippi River system. Although some dams have been constructed, most of the management relies on dikes and levees. The fundamental difference between dikes and levees is:
Dikes: Always hold water back.
Levees: Only restrain water during floods.
Both structures are linear and line the riverbanks rather than blocking the river like dams do. The approach to managing the Mississippi differs from that of Egypt due to varying climates and water source availability. Navigation and flood control are prioritized over a year-round water supply since we are fortunate to have numerous water sources. In downstream areas, the primary goal of the Mississippi River's dike and levee system is flood control, but this comes with significant repercussions.
Consequences of Flood Control
The long-term management of the Mississippi has caused the river's delta to build out dramatically into the Gulf of Mexico. However, this process has come at a substantial cost, as it has led to the cessation of sediment deposition across a broader area of the coastline, resulting in many coastal areas, including the land beneath New Orleans, slowly sinking.
Impact on New Orleans
To keep the ocean out, New Orleans has been lined by dikes. This reliance on flood control measures was tragically highlighted during Hurricane Katrina in 2005, when the storm surge breached these dikes, leading to significant flooding across the city and resulting in approximately 1,800 fatalities. Beyond this event, the city faces continual flooding risks due to heavy rain. Much of New Orleans lies several feet below sea level, necessitating a complex system of canals and pump stations to collect rainwater and pump it back into the sea. If rainfall exceeds pump capabilities, flooding is inevitable.
Infrastructure and Elevation Issues
The low elevation of New Orleans explains why its cemeteries are above ground. When Hurricane Katrina struck, seawater flooded the city, which couldn't be evacuated until the dikes were repaired, leaving parts of New Orleans under water for weeks afterward. The question arises: how did New Orleans come to be primarily below sea level? The answer lies in the decision to control floods via levees and dikes.
Historical Development of the Riverbanks
When Euro-Americans began settling along the Mississippi, natural levees formed from sediment deposition during floods. As floodwaters crested the riverbanks, sediment would be laid down, creating elevated areas that offered some protection against future flooding. Recognizing this, settlers, beginning with the French who founded New Orleans, decided to enhance these natural barriers by constructing artificial dikes or levees on top of them, thus creating even greater flood protection. While effective, this strategy had unforeseen consequences.
Consequences of Sediment Management
Due to the deposition of sediment in the channel rather than the floodplain, the height of the river increased, raising the flood risk and creating a cycle of increasing levee heights. Currently, in parts of New Orleans, the surface of the Mississippi River is up to 18 feet higher than the surrounding city. The need for ongoing flood prevention has led to a situation where levees temporarily raised by sandbags could cause additional issues downstream.
Problems with Levee and Dike Reliance
Relying on levees and dikes has several complications:
Insufficient Coverage: Ending levee construction prematurely or not extending them far enough upstream can simply push floodwaters to downstream areas, leading to higher flood levels.
Ineffective One-Sided Protection: Building levees on only one side of the river offers protection only for that side, exemplified by the case of Davenport, Iowa, which suffered flooding due to its choice to preserve views rather than build levees.
Construction and Maintenance Costs: The substantial costs associated with these structures have led to a reliance on the Army Corps of Engineers, which spends significant resources to maintain them. In some areas, 85% of barge traffic on the Mississippi is associated with the maintenance of levees, while only 15% is for commercial traffic.
Declining Agricultural Yields
The transition to a system of levees and dikes has also initially decreased agricultural yields. Although floods can devastate communities, they carry sediment vital for maintaining the soil's fertility in agricultural regions. Consequently, the absence of flood replenishment has led to increased dependency on artificial fertilizers.
Development of Marine Dead Zones
Artificial fertilizers used in the Upper Midwest create nutrient runoff that reaches the Gulf of Mexico, triggering algal blooms that ultimately lead to significant ecological consequences. This phenomenon results in oxygen depletion in the water, severely affecting marine life and fisheries. The Upper Midwest, particularly the Minnesota River valley, is a crucial contributor to this nutrient flow.
Tile Drainage Systems
Much of the agricultural land utilizes extensive tile drainage systems designed to remove excess water. Originally constructed from ceramics, modern systems are mostly plastic. These systems effectively drain fields but also transport fertilizers downstream, contributing to the Gulf's nutrient issues. This runoff creates a dead zone in the Gulf that devastates coastal fisheries, highlighting the interconnected nature of river management and agricultural practices thousands of miles away.
Channel Sedimentation and Wetlands Loss
The management practices of constructing levees and dikes also lead to plugging in the downstream river channel due to confinement of floodwaters. Sediment can only settle on the channel floor or at the mouth of the river, clogging navigation routes for large vessels. The Army Corps of Engineers also allocates significant resources for dredging these sediments due to the consequences of their own management efforts.
Moreover, saltwater incursion due to the sinking coastline has resulted in the degradation of freshwater wetlands. Healthy wetlands can collapse under saltwater pressure, leading to ecosystem collapse. An innovative project now pumps rainwater from New Orleans streets to combat the effects of saltwater intrusion on freshwater ecosystems.
Hurricane Katrina and Long-Term Maintenance Responsibilities
Long-term maintenance and mismanagement of the river system have substantially contributed to the Katrina disaster. The Mississippi River delta, formed from sediment transported by the river, undergoes compaction and subsidence over time, affecting the stability of coastal land. Levee construction has constrained the river to its current course, leading to uneven sediment deposition and exacerbated subsidence of coastline areas, further increasing the risks associated with flooding.
The system is currently at risk as the Mississippi River threatens to change course due to the subsidence of the western coastline, prompting the Army Corps of Engineers to construct barriers to keep the river in its current course. This intervention may only be a temporary solution to an enduring crisis.
Accountability and Future Considerations
The primary cause of our current flood management predicament is the decision to prioritize flood control through levees and dikes, which has led to a lack of sediment deposition necessary for maintaining coastal integrity. The Army Corps of Engineers' policies stem from congressional mandates following historical floods, demonstrating a collective responsibility on the part of the American public. Transitioning to natural river behavior would either necessitate intentional episodic flooding to deposit sediment or allow the river to abandon its current course, demonstrating the need for systemic reevaluation of flood control strategies.
In conclusion, the situation presents a dire need for revisiting how we live along the Gulf, drawing lessons from areas like the Netherlands where innovative housing designs adapt to similar challenges. However, socioeconomic disparities complicate the implementation of such solutions, making the future of New Orleans precarious as the city continues to sink amidst looming flood risks.