The boundaries that separate streams in adjacent drainage basins are called

perimeter/drainage divide

Explanation:

The drainage basin of a stream represent an area of land from which the water is flowing toward the stream. This area has its own natural boundaries, which are usually highlands that alleviate themselves from the three sides relative to the stream which is located in the lowest part of the area.

The boundaries are represented with highland usually, as mentioned, running along ridges, mountains or hills. The water that falls into the naturally enclosed area all flows down toward a single stream directly, or to several streams, and than into the largest, central stream in the drainage basin.

Some of the drainage basins are very small, only occupying few square km, while some are enormous are larger that the majority of the countries in the world.