One may own the river bed but one does not own the water the boat floats on. Nor are the public/householders generally empowered like CaRT to manage the water course/levels.
Just because the red line of ownership is NOT shown on the Title Plan in the river centre (only marked as the bank line), common law allows that the landowner owns to the centre of the river.
Land boundries move with natual river realignments under the legal doctrine of accretion and diluvian - despite what the Title Plan shows, the river moves, the boundary moves. This makes some Title Plans look incorrect. And also may appear to show "unregistered land" which is in fact, not unregistered.
Get legal advice from a specialist.