Around 300 B.C., prehistoric Indians entered the Salt River Valley, where they developed an extensive Hohokam canal system and raised corn, beans, squash, agave, and cotton. More than 500 miles of Hohokam canals have been recorded in the valley, and estimates suggest they may have supported between 30,000 and 60,000 people with up to 100,000 acres under cultivation. Some canals extended more than 12 miles in length. For reasons still unknown, the Hohokam abandoned their canal system and left the valley by 1450 A.D. In 1878, Mormon pioneers constructed their first major canal by cleaning out an ancient Hohokam canal. The Park of the Canals preserves about 4,500 feet of ancient canals and is recognized as an important Native American heritage site and as a National Historic site, while the Hohokam canal system received an Award for Excellence in Prehistoric Engineering from the American Society of Civil Engineers, the first such honor given to an ancient civil engineering project.