As an experiment, I suggest that once you have flowed into it like the
rush of a river (excellent metaphor, by the way!) and feel that you have
filled it, then let go of the 'you' / 'it' polarity and allow yourself
to become at-one-with it. One way of making that transition from
"filling it" to "being it" is to use the emptiness of mind. In other
words, at the point when you feel you have "filled it", enter into an
emptiness of mind, releasing all thoughts entirely, and just BE.
When you enter into the emptiness of mind from the starting place of a
deep one-pointed focus, the point of focus has the effect of shaping the
emptiness. It's like walking down a path that leads through a gate to a
garden where you find the *essential meaning* of the thing you have been
focusing upon.