The Mississippi

Where it is

The Mississippi drains most of the interior of North America — a basin reaching from the Rocky Mountains to the Appalachians, covering a large share of the continental United States and a slice of Canada. Its major tributaries are themselves continental rivers: the Missouri from the west, the Ohio from the east. Below their confluences the river carries an enormous sediment load south across a wide alluvial valley to a delta on the Gulf of Mexico.

The land and water around it

• Mississippi River Deltadelta

  The wetlands at the river's mouth, built over millennia by sediment the river spread across them during floods. Levees now send that sediment down the channel and off the continental shelf instead, so the marsh compacts and erodes without being replaced.

• The Atchafalayariver

  A shorter, steeper route to the Gulf that the Mississippi has been trying to switch into for decades — the ordinary behaviour of a delta river seeking the steepest path. A control structure at Old River holds the split at a fixed ratio, which is a permanent commitment rather than a solved problem.

• The alluvial valleybasin

  A broad, flat floodplain of deep river-laid soil through the lower basin, agriculturally rich for the same reason it floods.

Its history, in short

Before railways, a river basin this size was a transport system, and the settlements at its confluences and its mouth captured the traffic. That is why Cahokia stands near the Mississippi–Missouri junction and New Orleans near the mouth. The modern history is one of control: levees to stop the river flooding, revetments to stop it moving, and a structure built to stop it changing course altogether. Each of those decisions worked, and each transferred a cost — chiefly the loss of the coastal wetland the river used to rebuild.

Timeline

Dates are shown on a timeline rather than as changing shapes on the map. The globe draws modern boundaries only, and tinting a modern outline to stand for a historical one would state something we cannot source.

Places that carry meaning

• Cahokiac. 1050–1350

  Earthen mounds on the floodplain near the confluence, including one of the largest earthworks in the Americas. Its scale is evidence of organised labour and long-distance exchange along the river system.

• New Orleans1718 onward

  Near the mouth, on ground that is mostly at or below sea level, protected by levees and pumps. The city's existence is a continuing decision rather than a settled fact.

Learn the language here

1. 1Understand which languages are spoken hereThe Mississippi is multilingual. Start with which languages are used, by whom, and for what — the phrase list only covers one of them.Written by Globe Quest

Check yourself

• How do levees contribute to coastal land loss?

  They stop floods spreading sediment onto the marsh, so it compacts and erodes unreplaced

  A delta stays above water because floods resupply it. Confine the flood and the sediment goes offshore instead.

• What is the Old River Control Structure for?

  Holding the flow split with the Atchafalaya so the river cannot change course

  The river has been trying to switch to the shorter Atchafalaya route; the structure prevents that indefinitely.

Worth arguing about

• The levees protect the cities and dissolve the coast. Over what time horizon should that trade-off be judged, and by whom?
• Holding the river in its present course is a permanent commitment. What else does your society maintain that it can never stop maintaining?

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