First, let's talk about the Indus Valley civilization. The Indus River would flood the land with nutrients twice per year, super reliably. Because of this regular flooding, the Indus Valley contained the most available calories per acre of any river of its time. Things grew easily, and the civilization flourished around 3000 BCE. A total of 1,500 sites have been found, and so the Indus Valley civilization was huge, containing many astonishingly impressive cities. These cities contained multi-story buildings, built from a uniform size of brick. Cities were oriented to catch the wind and contained extensive sewer systems that would have been the envy of much later civilizations. Indus Valley civilizations thrived through trading cotton and cloth with Mesopotamia, in exchange for things like bronze. Their reliable and plentiful land made them a peaceful people - few weapons have been found, suggesting they had what they needed. Unfortunately, it's also quite likely that they themselves were conquered as part of their decline.
Mesopotamia was another highly successful river civilization, but their river was quite different. The flooding of their river was extremely unpredictable, making proper irrigation complex and hard work. As a result, slavery was needed to irrigate effectively. Also probably as a result, their relationship with their gods was equally tenuous and unpredictable. The most powerful people in Mesopotamia were the priests, who were responsible for helping to avoid the wrath of the gods and bring success to the people. Trade was vital to Mesopotamia because, while their land was fertile, there was little metal, wood or stone. Mesopotamia was the world's first known territorial kingdom, developed through trade. But after drought, the weakened kingdom was invaded by pastoral nomads.