divisoria (or linea divisoria) de las aguas
I thought it better. Country limit arbitration is generally based either on the highest mountain peaks, or on the water shed division (for example, rivers that go into the Atlantic, or into the Pacific ocean). This would be correct for the context. Confluencia means something that meets. This line divides.
If the asker means the reunion of waters from the South and from the North, then confluencia is correct.
Local time: 07:58
Native speaker of: English, Spanish
PRO pts in pair: 2664