The Ants - 27:61
Who made the earth a dwelling place, and caused rivers to flow between them, and placed firm mountains on it, and placed a barrier between the two waters? Another god besides Allah? Verily, most of them do not know.
Why?
The Cave/18:32
Give them the example of two men: We gave one of them two vineyards, which We surrounded with date palms and planted other crops between them.
The Cave/18:33
We placed a river between them, so both vineyards yielded their fruit bountifully.