Mississippi River Flooding Case Study

Location and Context

The Mississippi River drains a basin covering approximately 3.2 million km² across parts of 31 U.S. states, making it one of the largest drainage basins in the world. Major tributaries include the Missouri River and the Ohio River, which meet the Mississippi upstream of St. Louis. Flooding is most severe in the middle and lower course, particularly in states such as Missouri, Arkansas, Mississippi and Louisiana.

Causes of Flooding

Natural Causes

• Large drainage basin
  Rainfall and snowmelt occurring hundreds of kilometres apart can combine downstream, dramatically increasing discharge.

• Spring snowmelt
  Snowmelt from the Upper Mississippi and Ohio River basins often coincides with heavy spring rainfall.

• Prolonged rainfall
  The 1927, 2011 and 2019 floods were caused by weeks or months of persistent rainfall rather than short storms.

• Low-lying floodplain
  South of Memphis, the river flows across a very flat floodplain, slowing flow and encouraging lateral spread of water.

Human Causes

• Levees and channelisation
  Over 5,600 km of levees speed up river flow, transferring floodwater downstream instead of allowing natural flooding upstream

• Loss of wetlands
  Over 80% of wetlands in Louisiana have been lost, reducing natural water storage.

• Urbanisation and farming
  Cities such as St. Louis and large areas of mechanised agriculture increase surface runoff.

• Climate change
  More intense rainfall events have increased the frequency of extreme floods, particularly since the late 20th century.

Impacts of Flooding

Social Impacts

• Local scale:
  During the Great Mississippi Flood of 1927, over 700,000 people were displaced, particularly in Arkansas and Mississippi.

• Regional scale:
  The 2011 flood forced the evacuation of communities along the lower Mississippi and closed parts of New Orleans’ flood protection system.

• National scale:
  Flooding has caused loss of life, including around 250 deaths in 1927, leading to nationwide concern over river management.

Economic Impacts

• Local scale:
  Homes, farms and businesses are damaged; in 2019, thousands of hectares of farmland remained underwater for months.

• Regional scale:
  River transport on the Mississippi, which carries around 60% of US grain exports, is disrupted during floods.

• National scale:
  The 2011 flood caused an estimated $2–4 billion in damage.

• Global scale:
  Disruption to US grain exports affects global food prices and supply chains.

Environmental Impacts

• Positive:
  Floodwaters deposit nutrient-rich alluvium, improving soil fertility on floodplains.

• Negative:
  Flooding spreads agricultural chemicals and industrial pollutants into wetlands and the Gulf of Mexico, worsening water quality.

• Long term:
  Loss of wetlands and riverbank erosion reduces biodiversity and increases future flood risk.

Flood Management Strategies

Hard Engineering

• Levees and embankments
  Protect towns and farmland but increase flood peaks downstream.

• Flood relief channels and spillways
  The Morganza Spillway and Bonnet Carré Spillway divert water away from New Orleans during extreme floods (used in 2011 and 2019).

• Dams and reservoirs
  Large dams on the Missouri River regulate flow but cannot fully prevent extreme floods.

Soft Engineering

• Floodplain zoning
  Limits development in high-risk areas, particularly in parts of Louisiana.

• Flood forecasting and warnings
  Managed by the National Weather Service, allowing early evacuation.

• Emergency planning
  Improves response times and reduces loss of life.

Sustainable Management

• Wetland restoration
  Re-establishing wetlands increases natural water storage and slows runoff.

• Allowing controlled flooding
  Some floodplains are allowed to flood naturally, reducing pressure downstream.

• Integrated river basin management
  Recognises that managing flooding requires coordination across the entire basin, not just individual states.

Overall Evaluation

Flooding on the Mississippi River cannot be prevented due to the river’s size and natural processes. Hard engineering has reduced risk to major cities such as New Orleans, but often transfers flood risk downstream. Increasing use of sustainable strategies, such as wetland restoration and controlled flooding, is essential to reduce long-term flood risk while working with natural river processes.