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Collapse: Why do civilizations fall?
Collapse: Why do civilizations fall?Mesopotamia

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Mesopotamia was known as the land between two rivers, the Tigris to the north and the Euphrates to the south. Rains were seasonal in this area, which meant that the land flooded in the winter and spring and water was scarce at other times. Farming in the region depended on irrigation from the Tigris and Euphrates Rivers. Many resources in Mesopotamia were scarce or absent, which stimulated trade in the region in ancient times. Supported by lucrative trade with its neighbors, Mesopotamia grew to become a powerful empire.

Mashkan-shapir was a typical Mesopotamian city, located about 20 miles from the Tigris River and connected to the river by a network of canals. Despite a flourishing civilization, Mashkan-shapir was abandoned within only 20 years of its settlement. What could have caused this rapid demise?

Poisoned fields: A contributor to collapse

Along with factors such as war and changes in the environment, scientists now believe irrigation techniques played an important role in Mashkan-shapir's collapse. The same process that allowed farming in this region also eventually made it impossible to farm. Irrigation has a Catch-22: if irrigation water is allowed to sit on the fields and evaporate, it leaves behind mineral salts; if attempts are made to drain off irrigation water and it flows through the soil too quickly, erosion becomes a problem. Scientists believe that Mashkan-shapir's collapse was caused in part by destruction of the fields by mineral salts. When mineral salts concentrate in the upper levels of the soil, it becomes poisonous for plants.

In Mesopotamia, irrigation was essential for crop production. The rivers were higher than the surrounding plain because of built-up silt in the river beds, so water for irrigation flowed into the fields by gravity. Once the water was on the fields, it could not readily drain away because the fields were lower than the river. As the water evaporated, it not only left its dissolved mineral salts behind, but also drew salts upward from lower levels of the soil. Over time, the soil became toxic and would no longer support crops. By about 2300 B.C., agricultural production in Mesopotamia was reduced to a tiny fraction of what it had been. Many fields were abandoned as essentially useless. Mesopotamian cuneiform tablets tell of crop damage due to salts.

Salt-damaged fields in California's San Joaquin Valley.

Could this happen today?

In the United States, California's San Joaquin Valley faces irrigation problems that are similar to those faced by Mashkan-shapir. The irrigated soil is becoming increasingly salty, as is the water table. Without irrigation, abundant crop yields would have been impossible in this arid area. With irrigation, the land will very likely become impossible to farm.

Modern methods don't seem to be helping the San Joaquin Valley avoid this fate. Farmers have tried to cleanse the salts from the soil by flushing it with water and draining it into the sea. They have tried pouring the salty runoff from irrigation down drains dug deep into the ground. So far, these solutions have not worked, and the fields closest to the water table are becoming poisoned by salts. Right now, it looks as if the San Joaquin Valley is headed for the same fate as Mashkan-shapir.

How much can we learn from archaeological evidence? Find out more in "Finding and Interpreting the Evidence."

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