Chasing the Mud: The Mississippi River Flood of 2011

Summary

The 2011 Mississippi River flood, one of the century's most devastating, prompted the opening of the Morganza and Bonnet Carre Spillways to prevent catastrophic levee failure. This action increased sediment flow into the Atchafalaya River, nourishing wetlands. Scientists studied sediment deposition, crucial for wetland health, using helicopters to access remote sites. They found varying sediment levels, with more near the Atchafalaya basin and less in distant marshes. This data is vital for wetland restoration in the face of sea-level rise.

Takeaways

• 🌊 The 2011 Mississippi River flood was one of the largest in a century, rivaling the floods of 1927 and 1993.
• 🚧 To prevent catastrophic levee failure, the Morganza and Bonnet Carre Spillways were opened, causing intentional flooding in some areas.
• 🌐 The flood increased water and sediment flow significantly in the Mississippi and Atchafalaya Rivers.
• 🔍 Scientists studied the flood to track sediment movement and its impact on the delta's marshes and swamps.
• 🌱 Sediment is crucial for wetland health, providing nutrients and helping to maintain land elevation.
• 🏰 Levees built in the 1920s for flood control have starved the wetlands of sediment, contributing to the delta's subsidence.
• 📉 From 1932 to 2010, an area equivalent to Delaware state size was lost due to marsh drowning and subsidence.
• 🌿 The opening of the Morganza spillway diverted more sediment into the Atchafalaya basin, potentially benefiting the wetlands there.
• 🧪 Scientists collected sediment samples to measure deposition and analyze sediment characteristics.
• 🌿 Plant roots help stabilize wetlands by binding soil and preventing erosion, while also slowing water flow to promote sediment deposition.
• 🌊 Sea-level rise exacerbates the need for sediment to maintain wetland elevation and prevent submergence.

Q & A

What were the Morganza and Bonnet Carre Spillways opened for during the 2011 Mississippi River flood?

The Morganza and Bonnet Carre Spillways were opened to reduce stress on the river levees and control structures, preventing catastrophic failure of the levee system and a possible change in the course of the river.

How did the opening of the spillways affect the Mississippi River delta?

The opening of the spillways caused flooding in some areas but allowed more water and sediment to be diverted to the Atchafalaya basin, which fed into the swamps and marshes, potentially helping to maintain soil elevations and wetland health.

What is the significance of sediment for the wetlands of the Mississippi River delta?

Sediment is crucial for the wetlands as it nourishes the plants, renews the land, and helps build the elevation of the soil surface, which is essential for the wetlands' health and stability.

What happened to the wetlands along the Mississippi River after the levees were built in the 1920s?

After the levees were built, overbank flooding was prevented, but the wetlands were starved of sediment. This, combined with subsidence and sea-level rise, led to the gradual drowning of marshes and a significant loss of land area.

How did the 2011 flood event impact the sediment deposition in the wetlands?

The 2011 flood event increased the flow of water and sediment, leading to several centimeters of new sediment being deposited in the freshwater marshes of the Atchafalaya basin, which is beneficial for the wetlands' stability.

What methods did the scientists use to study sediment deposition in the wetlands?

Scientists used helicopters to reach remote study sites and collected samples of the surface layer of sediment, measuring the thickness of the deposit and analyzing the sediment characteristics in the lab.

Why are plant roots important in the context of wetland stability?

Plant roots are important because they bind the soil, preventing erosion, and also slow the water, which causes sediment particles to drop out and deposit along the banks, contributing to the elevation and stability of the wetlands.

How does the salinity level affect the sediment deposition in the marshes?

As the influence of freshwater decreases and salinity increases, as seen in the salt marshes dominated by smooth cordgrass, less sediment from the flood event reaches these areas.

What is the role of the Mississippi River Birdsfoot Delta in the context of the script?

The Mississippi River Birdsfoot Delta is significant because it is located at the terminus of the river and is shaped like a bird's foot. It consists of extensive freshwater marshes that are kept fresh by the high flow of water from the river, allowing freshwater plant species to colonize.

How does marsh stability relate to the balance between sedimentation and subsidence?

Marsh stability is maintained when the rate of sedimentation on the soil surface balances the rate of compaction below the surface. This counterbalances subsidence and helps the marsh elevation stay level with the prevailing water level.

What was the purpose of the survey conducted by the scientists along the Louisiana coast?

The purpose of the survey was to understand the effects of river flooding on wetlands and to develop better ways to restore deteriorating wetlands in the Mississippi River delta by measuring sediment deposition at various locations.