Thanks for the question.
Well, first off, it says “must consider”, not “defined”, so it's a part of the consideration of how the laws apply.
Yes, some rivers are very long. Some of the longest rivers in the world are in this country, and there are also sources of water that are quite small. I think the concept is that there must be—not to define a term with the term—a connection to that water in order for a law to be passed.
In the same way that exists in the provincial and territorial concept, the Province of Ontario cannot pass a law over waters in Alberta, because there's no connection to those lands or to that watershed. I think it's talking about aligning laws over a space that is permeable and connecting those to a first nation.