In the ancient world, why did civilizations flourish around great river valleys?

This was an order issued during the civil war by president lincoln ending slavery in the confederate states

How did the voting rights act and the 15th amendment combine to improve individual rights for african americans

Before leaving their ship anchored in cape cod bay, the pilgrims agreed to the terms established by which of the following: a) the fundamental orders of connecticut b) the declaration of independence c) the mayflower compact d) the constitution