界河

The river is the boundary between the two countries.

这条河是两国的界河。

Rubicon was a small stream that flowed into the Adriatic and in Roman times marked the boundary between Cisalpine Gaul and ancient Italy.

在罗马时期，卢比肯河是一条流入亚得里亚海的小河，也是高卢与意大利的界河。