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The Mississippi-Atchafalaya River Basin (MARB)


The Mississippi River originates as a tiny outlet stream from Lake Itasca in northern Minnesota. During a meandering 2,350 mile journey south to the Gulf of Mexico, the Mississippi River is joined by hundreds of tributaries, including the Ohio and Missouri Rivers. Water from parts or all of 31 states drains into the Mississippi River, and creates a drainage basin over 1,245,000 square miles in size. Before reaching the Gulf, the Mississippi meets up with its distributary, the Atchafalaya River.

The Mississippi-Atchafalaya River Basin (MARB), which encompasses both the Mississippi and the Atchafalaya River Basins, is the third largest in the world, after the Amazon and Congo basins. Parts or all of 31 states plus two Canadian provinces drain into the Mississippi River, totaling 41% of the contiguous United States and 15% of North America. Along with being the largest U.S. drainage basin, the Mississippi also creates borders for 10 states. The Mississippi River provides necessary resources to the U.S. and has helped to shape American history and commerce, including tourism and the fishing industry.

Prior to the Louisiana Purchase, the Mississippi River acted as the western border for the United States. The water way was first used for trade with Indian tribes when fur pelts were floated down the river from Ohio. Once steamboats were invented, the Mississippi River became an important mode of transportation that revolutionized river commerce. Manmade locks and dams were created to control flooding and create deeper waters for steamboats. However, this system made it more difficult for water to be absorbed and made flooding even more detrimental. The convenience of a trustworthy mode of transportation and a constant water supply encouraged agriculture, industries, and cities to spread to areas along the river. Productivity from these areas resulted in large amounts of nutrients being discharged from the river system into the Gulf of Mexico. These nutrients have contributed to hypoxia.

Old Mississippi River Control Structure

Image courtesy of The Historic New Orleans Collection Accession No. 1999.111.34

The natural capacity of the MARB to remove nutrients has been diminished by a range of human activities. The Mississippi is one of the most heavily engineered rivers in the United States. Over time, the character of the old river meanders and floodplains have been modified for millions of acres of agriculture and urbanization. Many of the original freshwater wetlands, riparian zones and adjacent streams and tributaries along the Mississippi have been disconnected from the river by levees and other engineering modifications. This has caused a loss of habitat for native plants and animals and has reduced the biological productivity of the entire river basin. Historically, the coastal marshes of Louisiana have provided a natural barrier against the erosion caused by the fierce storms which often come from the Gulf by neutralizing some of the flow energy of the water. This capacity has been reduced by channelization.

Over the years, traffic on the river has caused increased bank erosion, turbidity, sediment resuspension, and disruption of native species habitats. The increased amount of river dredging, levee building, and construction that comes along with this traffic impairs aquatic life in many ways by disturbing their habitat.

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MARB Facts

Download a fact sheet (PDF 2pp, 2.3MB), based upon the EPA Wadeable Streams Assessment, that discusses nutrient pollution in the MARB.

Basin Area:

  • The basin covers more than 1,245,000 square miles
  • Parts or all of 31 states and 2 Canadian Provinces
  • 41% of contiguous United States
  • 15% of North America

River Size:

  • The Mississippi River runs 2,350 miles (3,781 km) from its headwaters at Lake Itasca, Minnesota to the mouth of the Gulf of Mexico
  • The width of the Mississippi ranges from 20 feet at the headwaters, to about 1 mile across in Louisiana

Water Discharge Rate:

  • Water is discharged into the Gulf of Mexico at an average rate of 600,000 cubic feet per second
  • It takes about 90 days for water to travel from the headwaters in Minnesota to the Gulf of Mexico
Picturesque Wetland

Wildlife Facts:

  • 260 fish species
  • 145 amphibians and reptiles
  • 50 mammals
  • 38 mussel species
  • 60% of North American birds use the Mississippi River as their migratory flyway

Other Facts:

  • 60% of grain exported from the United States is transported and shipped from the Mississippi River
  • The Mississippi carries an average of 436,000 tons of sediment each day
  • More than 50 cities rely on the Mississippi for daily water supply
  • The Port of South Louisiana, which stretches 54 miles along the Mississippi River, is the largest tonnage port district in the United States, moving some 233 million short tons of cargo in 2008.
  • Missouri's New Madrid earthquakes in 1811 and 1812 caused the Mississippi River to flow backwards

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