Apologies for my previous question, it might have been built on a wrong belief, which I hope will be clarified by answers to this question.
Few years ago, when the dispute between Egypt and Ethiopia started about the utilisation of the Nile river, I remember hearing/reading something that there is an international law that says something along these lines:
"When a river is located in more than one country, but only one/some of them using it, if no other country utilises it for X number of years, then the river will be owned by those of which have been using it for these years"
So I searched for what could support or oppose that, with no luck. Now I feel that either there is no such law about ownership of rivers that are located in multiple countries, or maybe me or whoever said that misinterpreted an exited law that doesn't exactly say that.
By countries owning the river I mean no other country can use the river's water in a way that would affect those countries without their agreement.