Water is a valuable resource. Rivers and lakes often serve as the boundaries between different countries. Both countries may want to use 100% of the water for their own use. How do they work out how much water each country gets to take from the body of water? For something like states in the US, there will be some federal board, regulations, or water rights granted which work out how much water each state can get. But when two independent countries both have borders along the same body of water, who gets to say how much water each country can take?
For example, the Rio Grande runs along the border of US and Mexico. That part of the world is very arid. I’m sure both countries would like to use all the water for their own use for irrigation. What’s to stop Mexico or the US from building a trench at their border and directing all of the water into their own country?
Specifically for the Rio Grande, around 80% of the water is already diverted for use. There is a treaty between the US & Mexico that covers usage and another water usage law that covers New Mexico vs. Texas.
In general, shared water sources have treaties covering their use. Great Lakes is another US example. This one with Canada of course.
There is no general rule of international law allocating such resources. It’s dealt with by means of agreements specific to the river in question concluded between the riverine countries, and in the absence of such an agreement, it can be a major bone of contention.
Not necessarily. Alabama, Georgia, and Florida have been fighting a “water war” since 1990, that was only settled in April of this year. The states were arguing over water allocations from the Chattahoochee River - Flint River basin, which rises in Georgia, forms the border with Alabama, and empties into Florida’s Apalachicola Bay. Florida claimed that Georgia was taking too much water for metro Atlanta, to the detriment of the Apalachicola Bay oyster industry. Ultimately, this was decided by the Supreme Court, who ruled in favor of Georgia.
Sometimes there is no meeting of the minds of who’s is who’s. The US and Canada have a small section of ocean in the Atlantic and a big section of the Arctic ocean which are unresolved however at this point it’s not a big deal as it’s not yet a useful section to make a stink over. The Arctic section however is getting some hub-bub as the ice caps melt away and oil resources are more accessible. But until it actually matters there is generally no reason to pursue it.
Also IDK about water sovereignty specifically but a interesting aside, in boarder disputes Google maps sometimes will show 2 different versions of where they boarder is and perhaps what that land is called depending on what side of the border you are on when you view it.