Full Idea
A river is a process through time, and the river stages are its momentary parts. Identification of the river bathed in once with the river bathed in again is just what determines our subject matter to be a river process as opposed to a river stage.
Gist of Idea
A river is a process, with stages; if we consider it as one thing, we are considering a process
Source
Willard Quine (Identity, Ostension, and Hypostasis [1950], 1)
Book Reference
Quine,Willard: 'From a Logical Point of View' [Harper and Row 1963], p.65
A Reaction
So if we take a thing which has stages, but instead of talking about the stages we talk about a single thing that endures through them, then we are talking about a process. Sounds very good to me.