“Says who?”
The law in most, if not all states.
Many of the Water Laws in the US go back far more than 100 years and have not been altered. In law school, there are classes on Water Law. There are attorneys who make a great living specializing in Water Law.
The state, local governments and the various river authorities consider the water to belong to them, and usually the law is on their side.
You completely missed the point.