Each country does indeed ...

Each country does indeed write its own laws, but maybe it also helps to know there can be big differences between countries. Some countries have no legislation, but in others the entitlement to use water is covered by property legislation. In the UK, water in river courses cannot be "owned", so is not subject to property laws. Instead it is covered by environmental legislation which requires any abstraction from the river (including a complete diversion) to be allowed by an abstraction licence, which can only be obtained in the first place from the Environment Agency. Otherwise, any interference with the flow of a river is illegal.