River Case Study: The Mississippi

• –  Length – 3,800 km
• –  Width – Flood Plain is 200 km wide at its widest point
• –  Releases 600 million tonnes of sediment of load each year (into the sea and along its course)
• –  Flows through 10 states
• –  Carries 13% of all freight traffic in the USA

1. What Economic Opportunities are Offered by The River Mississippi?

–  11 million tourists each year in the upper reaches of river

–  tourism contributes $1.2 Billion to the economy each year

–  Sports Fishing – $100 milling, Waterfowl hunting – $58 million

–  29 power plants provide energy to vast array of factories and homes

–  Water from river used for cooling in 80% of Energy Production Facilities

–  Silt deposits formed on huge flood plains made area around the river among the most fertile

regions of the USA (sugar canes, rice, tobacco, and cotton plantations)

–  River provides main transport routes for exports around the world

–  Today over 100 million of commodities transported down the river every year (including 56%

of the nation’s corn, and 41% of soybean exports)

2. What problems/hazards are Presented by the Mississippi?

–  Despite efforts there has been major floods over the last 50 years

–  Particularly severe flood in 1993 (see below)

–  Many people live along the river’s flood plains

–  Many scientists have argued that overflowing in a natural part of the river, and should be left to

happen (prevention measures should be abandoned)

–  One of the worst areas of flooding is the point when the rivers enters its delta at the Gulf of

Mexico and spreads through distributaries e.g.  Hurricane Katrina storm surge at the mouth of the Mississippi in New Orleans

–  Careful management is needed to stop the build-up of sediments causing even more floods

–  The river’s length of 3,800 km means that it’s very difficult to prevent flooding

– needs constant dredging to allow for ship traffic

Great flood of 1993”

• Between April and October 1993
• 30,000 square miles flooded
• Affected both the Missouri river (tributary of Mississippi) and Mississippi basins
• Caused by a series of storms with heavy rainfall (less time to absorb into the soil or  infiltrate) throughout period, in some places 750% more rainfall than usual with  rainfall amounts of (125 to 175 mm) in 24 hours
• Lower than average temperatures (less evaporation)
• By the beginning of June the ground was already saturated meaning that continued rainfall went straight into river systems
• St. Louis, river levels were nearly 20 feet (6 m) above flood stage.
• 700 privately built agricultural levees were overtopped or destroyed along the Missouri River.
• Navigation on the Mississippi and Missouri River had been closed since early July resulting in a loss of $2 million (1993) per day in commerce.
• Approximately 100,000 homes were destroyed as a result of the flooding, 15 million acres (60,000 km²) of farmland inundated
• 32 people died
• Approximately $15–20 billion in damages
• River levels breached levees and floodwalls (levees)

3. What Managements/Solutions Have Been Tried/Planned for The River?

https://www.bbc.co.uk/education/clips/z3bqxnb

Dams, spillways, artificially strengthened levees, river straightening, flood warning systems, afforestation

– 6 dams along the Missouri, 1600 km chain of 105 reservoirs: these hold back water until river levels have lowered and can manage more water

–  9 dams have been built along the Tennessee river (also a tributary)  

– Afforestation in the upper Mississippi drainage basin system: planting of trees increases interception and increases lag time

– The Bonnet Carré floodway (9 km long spillway) diverts excess water from the Mississippi, 50km north of New Orleans

– River straightening e.g. one  530 km stretch of river has been shortened by almost 300 km by cutting through meanders. This increases the gradient and speed with which flood waters can be carried away.

– Reinforced levees using concrete blocks: success in St Louis in 1993 as river levels almost reached the full height of levee but it just managed to hold them back.