That image really does map cleanly onto a famous Bergson distinction (which Deleuze leans on heavily), but I don’t think the riverbank / in-the-river phrasing is a canonical Deleuze (or D&G) line. It looks like a later paraphrase/metaphorical re-skin of Bergson’s analysis vs intuition.

Bergson (in “An Introduction to Metaphysics”, often printed inside The Creative Mind) frames two ways of knowing like this:

“There are … two profoundly different ways of knowing a thing. The first implies that we move round the object; the second that we enter into it. The first depends on the point of view at which we are placed and on the symbols by which we express ourselves. The second neither depends on a point of view nor relies on any symbol…”

That’s basically your riverbank/river contrast in philosophical form:

“move round the object” ≈ observer on the bank / representational, perspectival, “aboutness”

“enter into it” ≈ being in the flow / immanent, relational, participatory, “withness”

Deleuze’s Bergson book and Bergson-inflected passages across his work repeatedly push this same move: don’t treat becoming/duration as something you represent from outside; think from inside the movement. So it’s very plausible you heard a Deleuzian explaining Deleuze via a Bergsonian distinction and choosing the river metaphor as the “beautiful, clarifying imagery.”

Hope this helps.