The river forms a natural border between the two countries.

The river forms a natural line between the north and south sections of the city.

The river forms part of the boundary between the two states along its entire course.

A mountain or a river formed the border to our world.

For most of this course the river forms the boundary between the two countries.

The river forms a portion of the international border between the two countries.

In all three cases the rivers form an ideal supply line.

These three rivers (among others) also form the borders of several states.

The river forms the dividing line between north and south Morgan.

The river and its wetlands form the complex's southern boundary.