My opinion

The Harappans had more rain than the Mesopotamians and Egyptians, but they still relied on the silt of the floods to create the fertile soil. They also were plowing the soil as early as 2700 B.C.E.As such; they were able to combine their fertile soil, ample water, and the latest technology to produce the agricultural surplus necessary to develop a civilization. Plus, the Harappans had a flood plain, over 500, 000 square miles, which was three times that of Mesopotamia. 

The Egyptians thus figured out how to harness the flood waters of the Nile. The topography of the Nile River valley was much more uniform than that of Mesopotamia. Also, the population of early Egypt was more homogeneous than Mesopotamia’s. As a result, the Sumerians developed larger city-states in pockets of similar people and topography than did Egyptians.

Finally, Every culture is different and make different use of the environment and resources they have, the three cultures especially the Egyptians and the Sumerians knew how to use the rivers they had available and made the better of them.