Why were the earliest cities built by rivers?

Four main reasons:

1. For fresh water to drink, cook with, and clean bodies and clothing.

2. For the food to be found in water, like fish, crawdads, clams, and so forth.

3. Rivers were handy to dump sewage into and let it be carried away. This helped reduce disease among those people. Of course, anyone living downstream might not be so fortunate.

4. For transportation via canoe, raft, or boat.