The Three Sisters were usually eaten in unison, as they provide fairly balanced nutrition when consumed together(for example, beans and corn together provide a complete set of the essential amino acids). Native American farmers also employed irrigation. This technique was utilized throughout much of the Southwest and is useful where water is scarce. Irrigation was and is still used today throughout much of the world. Native Americans controlled the amount of water that reached their fields by building long irrigation canals to redirect water from a source to water their crops. The Hohokam people constructed about 600 miles of irrigation canals from AD 50 to 1450 near Phoenix, Arizona (Berzok 2005, 51). Part of the canal is used by the City of Phoenix today. The Olmecs of Mesoamerica built canals over 4,000 years ago. Chinampas, artificial islands constructed in swamplands and lakes, were invented by Mayan farmers and the technique became used extensively throughout Mesoamerica and was later used by the Aztecs as part of the land reclamation process of the city of Tenochtitlan. This technique increases arable land, and provided additional farming plots the population of Mesoamerica grew (Keoke and Porterfield 2005, 60). In the Northeast Woodlands and the Great Lakes region, an advanced society known as the Moundbuilders emerged (Berzok 2005, 13). This society lived in the flood plains of the Mississippi river basin. This culture farmed mainly maize. They had little need for foraging and grew to an advanced civilization due to food surplus. They were the largest civilization north of the Rio Grande. Native Americans also developed storage systems such as storage containers which allowed them to store seeds to plant during the next planting season. They also stored food in dug-out pits or holes in hillsides. Native Americans developed corn cribs. These were storage bins that were elevated off the ground. This technique prevented moisture and animal intrusion (Berzok 2005, 122-123). Selective crop breeding was also employed. Corn is a domestic plant and cannot grow on its own. The first corn grown by Native Americans had small ears, and only produced a few kernels per ear. By 2,000 years ago, single stalks with large ears were being produced (Keoke and Porterfield 2005, 55). Native Americans created over 700 varieties of corn by 1500 AD.