Full Idea
That will be the same river which flows from one and the same fountain, whether the same water, or other water, or something other than water, flow thence.
Gist of Idea
It is the same river if it has the same source, no matter what flows in it
Source
Thomas Hobbes (De Corpore (Elements, First Section) [1655], 2.11.07)
Book Reference
Hobbes,Thomas: 'Metaphysical Writings', ed/tr. Calkins,Mary Whiton [Open Court 1905], p.86
A Reaction
This makes the source the one necessity for a river. I think the end matters too. If the Thames reversed direction, and flowed into Wales, it would not be the Thames any more.