It's not just water rights. It's the mineral rights too! You are allowed to use the topsoil, but any passing stranger (or company) can stake a claim to lease the mineral rights from the appropriate government (State or Province) and start drilling or digging in your garden or cornfield. That is why establishing Sates was so important in the 19th Century. Ordinary people were trilled to be allowed to own their own land, but actually, they were (and are) just renting it from the State.