Word is not so hot. Best hope for a good ride is to wait for a thunderstorm upstream, and then ride it after a rise when the water clears. Where you rafting? The "loop" from the crossing below GSP to the old Bee's campground crossing is always pretty navigable.
I've never been to Choke Canyon before. My understanding is the river is usually mostly underground or seeps for most of the way from Knippa to the reservoir.
Actually Mouth I was being facetious. The river flows above ground through the brush country most years. But it's murky, slow, narrow, and full of log jams.
Great place catch whites from the bank in Feb/March under the Hwy. 16 bridge, though.