What would a fair way to share a river look like?

Rivers cross borders. Upstream countries, midstream farms, downstream cities and fishing communities all depend on the same water, and they often disagree about what is fair.

There are several principles in play. Prior use: whoever has used it longest has a claim. Need: those with the greatest need should get priority. Contribution: places where the water originates should have a say. Equitable use: each party's share should be reasonable given all the circumstances, including the environment itself.

We are asking readers to propose a fair arrangement for a specific, real or imagined river. A good answer would name the principles it favours and why, say how disputes would be settled, what data would be shared and how often, who would speak for the river's ecosystem, and what would happen in a drought.

Examples from basins you know are especially welcome, including cases where cooperation worked.