I understand your approach, but the parts are not connected on the spot, they flow into the channel. However, the canal is not part of these rivers, the canal drains into the North Sea via the locks in Brunsbüttel, there is only one weir as a further connection for water level regulation at the lock of the Giselau Canal.
That's why I wouldn't add the parts of the channel to the relations. But I'm not sure about that either, because I have too little to do with waterway relations. If you want to adjust these relations, I would suggest that you discuss this in the forum first.
https://community.openstreetmap.org/c/communities/de/56
German forum, you can write in English, it translates automartically.

Best regards puma515