Answer :
Four important river valley civilization of the ancient world would are Mesopotamia civilization, Egypt, Indus valley and Yellow river civilization. All these civilizations happened to be around similar period in history. The Mesopotamia civilization was settled around Tigris-Euphrates valley around 3500 BC. The Egyptian civilization in Nile valley started around 3200 BC, the Harappan civilization around the Indus valley would gain prominence around 3200 BC and the Yellow river civilization around 2100 BC.
