There is no Canadian law that says you can't do it. There's no law, but there is a Canadian policy that prohibits inter-basin diversions. In the scenario you're talking about, there wouldn't be an inter-basin diversion.
The Canadian policy talks about inter-basin diversions between the five major Canadian basins--Hudson's Bay, the Missouri, the St. Lawrence, and so on--but the scenario you're talking about would not be prohibited by any policy or law as far as I know--even if you did it.