Irrigation is a common farming practice, several thousand years old. It is an effort to add water to crops when rainfall is not enough to grow those crops. A North American group called the Hohokam lived 600-1600 years ago in the Lower Salt River valley in Arizona. They are well-known for their irrigation engineering. The Hohokam created the greatest series of canals in the prehistoric Americas north of Peru.
Archaeological and soil science studies tell us that the Hohokam were able to build more than 1,500 kilometers of canals. That’s about 932 miles, longer than the driving distance from Phoenix to Boise, Idaho. Irrigation canals built by the Hohokam were features that the first settlers in the Lower Salt River valley could see. Today, modern development has destroyed most, if not all the canal structures. An amazing feature of those prehistoric canals is that they were built without the aid of horses or cattle as energy sources. They were also built with stone tools rather than metal tools.
Unfortunately for the Hohokam, the water that runs in the Salt River is, well, salty. In addition, the Lower Salt River valley is within the hot-dry Sonoran Desert. Salt River irrigation water that is put on crops can evaporate. If the soil is not properly drained, there can be a buildup of salt in the soil. Salt accumulation on farm fields reduces the chance of growing healthy crops. Soil salinity is a problem that even modern-day farmers must manage. It can even be a problem in your own yard, or you nearby city park.
The salt present in the Salt River comes from springs in the eastern part of the watershed. The flow and evaporation of salty river water has occurred over time. This resulted in large underground salt deposits in the western portion of the Lower Salt River valley. These deposits are the result of the drying of a prehistoric lake and are mined for table salt and other commodity uses.
Salts are a problem for plants. They can tolerate only certain levels of soil salts in their root zones. High salt levels disrupt many metabolic processes and cause reduced plant growth or even plant death. Such impacts to plant growth caused reductions in food production. This adversely affected Hohokam farm communities. It is a likely contribution to the Hohokam’s eventual movement from the Lower Salt River valley.
The Hohokam supplemented their diets by also growing food in locations that were not irrigated. Studies have found areas like rock terraces where water from the occasional rains were collected to grow some additional food. The Hohokam also supplemented their diets with a wide variety of wildlife and wild plant species. These would come from the nearby riverine, riparian and Sonoran Desert habitats.
When the Hohokam first started farming in the valley, they could succeed. It was the accumulation of salts that harmed the soil – and the eventual crops. It’s possible that springtime floods even helped the farmers by flushing salts from the soil.
The salt content of the Salt River in flood stage would be very diluted. Sometimes flood waters washed over the riverbanks onto saline soils. Those flood waters could flush salts, from those fields that drained well. Such soils could once again be farmed.
Floods were thus both good and bad events. They were good if they flushed soil salts, enhanced soil fertility, and were not too destructive to canal systems.
Great floods, however, could be very destructive events. It was tough being a Hohokam farmer. Floods could destroy the canal systems. And, if the soil became too saline, the farmer would have to move to a nearby uncontaminated area. They would have to modify or build new canals, and farm that new land until it also became contaminated with soil salts.
The greatest difficulty for the Hohokam farmer would come from several consecutive years of drought. At those times there would be less water for irrigation. More salts would accumulate in the soil without the springtime floods. Perhaps the difficulties encountered in such drought periods would be the cause for the Hohokam to have eventually moved from the Lower Salt River valley.
Today’s crops can be grown in the Lower Salt River valley because of improved irrigation techniques. Salt River flood waters are stored in a series of reservoirs. Water for irrigation is efficiently delivered as needed. In addition, fields have been leveled, and drainage systems help move the leached salts through the soil and away from the crop root zone. Cultivation practices have improved.
An estimated 5.7 million hectares of land in the United States have been taken out of agricultural production or have experienced reduced crop yields because of soil salinity. Agricultural engineers continue to develop solutions to the problems of soil salinity, as we try to improve our re-use and recycle of limited water supplies.
By Henry L. Short
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This blog was written from a paper published in the Journal of Environmental Quality “Saline Soils and the Agricultural Failure of a Prehistoric Population,” by Henry L. Short, doi:10.2134/jeq2019.01.0015