Answer:
Not only do the river valleys provide food resources such as fishing, they also often have a wide, fertile floodplain readily suited for agriculture.
Step-by-step explanation:
Even the earliest, least advanced farming technology would have been successful in producing a substantial crop surplus from a river valley's naturally fertile soil. Agricultural innovations that had been developed elsewhere were introduced into the valleys of the river and developed on a larger scale. In Mesopotamia, Nile Valley, the Indus Valley, vegetation, and livestock thrived in the great river valleys in a quite significant form, especially after irrigation technologies were established to maximize the use of the region around the river.