The river is a way of talking, not a scene to imagine. You are not trying to see water, you are looking inward and noticing what turns up.

What the research says

In an eye tracking experiment with 43 novice dart players, deliberate self talk helped people hold their attention and their performance under distraction and mental fatigue, though how much it helped depended on how worn down they were. What was being trained there was where attention went, which is the same target here. It was a small study in a sports task, so it supports the idea of attention as trainable rather than this exercise in particular.

Strength of evidence: one line of research. A single study, or a theoretical account.