Why is the shape of the Mississippi delta different from the Nile delta?

Deltas have different shapes depending on how much sediment is deposited by the river compared to how much sediment is eroded and redeposited by waves and tides. A river dominated delta, such as the map below of the Mississippi delta, Louisiana, USA, is sometimes called a bird's foot delta because of its shape.



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One big difference between the two is that while the Nile delta is shrinking (the Mediterranean Sea is eating away more sediment than the river can supply), the Mississippi delta is growing. The image below shows the growth of the Mississippi delta over the past 10,000 years.

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The Nile has few tribu- taries. The Mississippi drainage system is highly branched. The Mississippi takes longer to crest and subside. Climate is the factor most responsible for the rivers' differences.

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Time, weather, and human intervention have all shaped the Mississippi Delta in Louisiana, a giant bird's foot shape protruding into the Gulf of Mexico.

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The Delta forms the most important bird and waterfowl migration corridor on the continent and supports North America's largest wetland area and bottomland hardwood forest. The Delta's cultural traditions are as rich and diverse as its natural resources.

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Rather, the Mississippi Delta is part of an alluvial plain, created by regular flooding of the Mississippi and Yazoo rivers over thousands of years. The climate is humid subtropical, with short mild winters, and long, hot and wet summers. The land is flat and contains some of the most fertile soil in the world.

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For 7,000 years, the Mississippi River has snaked across southern Louisiana, depositing sediment from 31 states and 2 Canadian provinces across its delta. As sediment accumulated under water, plant communities began to develop, trapping more sediment and building land.

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When compared to other world rivers, the Mississippi-Missouri River combination ranks fourth in length (3,710 miles/5,970km) following the Nile (4,160 miles/6,693km), the Amazon (4,000 miles/6,436km), and the Yangtze Rivers (3,964 miles/6,378km).

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Humans have upset the delicate balance of land gain and loss in the Mississippi River Delta. Dams, levees and channels along the Mississippi have prevented land-forming sediments from reaching the delta, and most of those that do are discharged deep into the Gulf of Mexico.

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The problem is the delta's wetlands were and still are built and sustained by sediment delivered by the river. Leveeing of the river cut the tie between the sediment-filled river and its delta, stopping the cycle of new wetland growth.

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