As cities emerged and populations grew in Mesopotamia, more water had to be provided to increase agricultural production. A. Early on, irrigation was recognized as a valuable practice, even though it was labor-intensive and brought with it problems of salinization and damaging floods. B. Levees were the major means of protection against flooding, while leaching with added water and lowering the water table helped to control salinization. C. Because of the enormous amount of labor involved in irrigating fields, farming was increasingly moved to hill slopes, where irrigation systems required less labor. D. The mountain water that was used to irrigate farmland in Mesopotamia was exceptionally high in salt, causing rapid salinization of the soil. E. The practice of leaving fields uncultivated periodically was used primarily by societies lacking a large labor force. F. As cultivation was extended to hill slopes, methods were developed to better retain water from rainfall for crops growing on hillsides.