A mountain ridge that separates river systems is called a watershed, or sometimes a divide.

Here's why:

* Watershed: This term refers to the entire area of land where all water drains to a common point, such as a river, lake, or ocean. Mountain ridges often act as boundaries for watersheds, directing water flow in different directions.

* Divide: This term specifically refers to the elevated area that separates different drainage basins. It's essentially another word for a watershed boundary.

So, both terms are correct, but "watershed" is a more general term that encompasses the entire drainage area, while "divide" specifically refers to the ridge itself.