Water flow produces natural levees and canals within riverbeds, but subsequent flooding can also change the direction of rivers, washing out the riverbanks and changing agricultural access to the surrounding plains. Using technological innovations, the early settlers of Sumeria built levees along the riverbank with baked-mud bricks and bitumen to prevent erosion during the high flood seasons. When the water levels were high, holes were made in the levees for water to flow out and irrigate the crops. During the dry season, advanced cultivation tools called shadufs were used to collect water in the low-lying area of the river and bring it to the agricultural plains. Subsequently, when the levee walls failed, Sumerians dug artificial channels to redirect the water. Artificial irrigation techniques were further mechanized with levers and pulleys, which also allowed for the magnification of human and animal force.
 Richard Cowen, “Chapter 17: Ancient Irrigation.” Exploring the Earth. Accessed January 7, 2018. http://mygeologypage.ucdavis.edu/cowen/~gel115/index.html