What is the name for a mountain range that separates river systems that travel in the opposite direction?

The name for a mountain range that separates river systems that flow in opposite directions is called a "watershed" or "divide." More specifically, it can be referred to as a "continental divide" when it separates river systems that drain into different oceans or seas. An example of this is the Continental Divide in North America, which distinguishes rivers flowing toward the Pacific Ocean from those flowing toward the Atlantic Ocean.