The city already owns those reservoirs, as well as the water tunnels that deliver the water into the city (which require little maintenance, as they run on gravity rather than pumps).
Of course, all of this is on the level of fantasy, because it's not going to happen.
The city may have ownership, but the local/state/national governments still have say on things like environmental protection. Owning a forest doesn't give you the right to clearcut every tree. Owning a reservoir does not give you control over where that water comes from (the watershed) nor any of the environmental or safety rules governing water.