How did river valleys trade with each other?

The rivers facilitated trade and allowed some of the cities to grow quite wealthy. China is geographically isolated. Although the Shang traded with other civilizations, their limited contact with others produced an ethnocentric outlook.

The River Valley civilizations came up with many things we take for granted such as calendars, the wheel, alphabets, bureaucracies, divisions of time into units of 60, and complex mathematical concepts like square roots.