Drainage basin or drainage system
Drainage basin are defined by drainage pattern, which is controlled by the underlying geology and landform.
more on what creates the pattern.
Think about this backwards: instead of thinking that civilizations happen to live in specific watersheds, consider that it is the mountains that force them to live in these separate watersheds. Rivers, like many early cultures, are "orographically constrained", topography will dictate how separate and distinct they become over time from one another....