Answer:
The river system that helped the world's earliest known civilization to develop was Mesopotamia.
Step-by-step explanation:
Mesopotamia is the name for the historic region lying between the Tigris and the Euphrates in the northeast of the Middle East. The historic nucleus of the territory is roughly equivalent to central Iraq today, but the name is also used to designate an area that includes Iraq, eastern Syria, parts of southeastern Turkey and southwestern Iran. It forms the eastern end of the Fertile Crescent and is known as the cradle of civilization. The land between the two rivers and their banks is extremely fertile because it is alluvial; every time those rivers overflow they are left behind by alluvium.
The first people to capitalize on this were the Sumerians, who settled in Mesopotamia around 4,000 BC to cultivate the fertile land and grow crops. They founded one of the first civilizations in world history that flourished in the city-states of Ur, Kish, Uruk and other sites.