give two reasons why there are many large cities in the northern plains
Indo-Gangetic Plain The Indus-Ganga belt is the world's most extensive expanse of uninterrupted alluvium formed by the deposition of silt by the numerous rivers. The plains are flat and mostly treeless, making it conducive for irrigation through canals. The area is also rich in ground water sources.The plains are the world's most intensely farmed areas. The main crops grown are rice and wheat that are grown in rotation. Others include maize, sugarcane and cotton. The Indo-Gangetic plains rank among the world's most densely populated areas with a total population exceeding 400 million.
Cahokia At the high point of its development, Cahokia was the largest urban center north of the great Mesoamerican cities in Mexico and Central America. Although it was home to only about 1,000 people before circa 1050, its population grew rapidly after that date. According to a 2007 study in Quaternary Science Reviews, "Between AD 1050 and 1100, Cahokia's population increased from between 1400 and 2800 people to between 10,200 and 15,300 people".[18] an estimate that applies only to a 1.8-square-kilometre (0.69 sq mi) high density central occupation area.[19] Archaeologists estimate the city's population at between 6,000 and 40,000 at its peak,[20] with more people living in outlying farming villages that supplied the main urban center. In the early 21st century, new residential areas were found to the west of Cahokia as a result of archeological excavations, increasing estimates of area population.[21] If the highest population estimates are correct, Cahokia was larger than any subsequent city in the United States until the 1780s, when Philadelphia's population grew beyond 40,000.[22] Moreover, according to the same population estimates, the population of 13th-century Cahokia was equal to or larger than the population of 13th-century London.