Doesn't seem to be the problem in this instance. I have changed it.
But yeah, a river is an abstract concept, one can be part of another, or two rivers might even overlap partially...

As for the tunnel segments, I've realized that the optimal way to make Waterways is to create one river Line object, name it and make it a new relation, and then expand it by rightclicking and using the Continue and/or Split functions. That way, each new Line automatically given the name and relation memberships of the original object.
A Waterway relation is much easier to deal with than 3 Lines, and a Multipolygon relation is handier than an equivalent decagon (10-sided) Area.

(I am going through most of the named waterways I can find on Zealand and putting them into Relations. When this is done, maybe the administrators can use this to study how Waterways work, somehow.)