Used to fish there a fair bit in high water . Saw it come up fairly rapidly at times but never even close to what I would call a flash flood .
People that say the canyon section may be dangerous may be referring to the possibility of a flash flood from the unstable landslide letting go? Not sure. I've been on it in heavy rain, and I would say it's far from a flash flood scenario, you have ample time to leave.
I saw a few in Arizona and other places in the states, and they are nuts. You get rain and literally 5 minutes later there is a river half the size of the Vedder where there was nothing before that will take a car no problem. The standard protocol in one area we were at was to pull over in the middle of the road with 4 ways on (during the heavy rain you couldn't drive), and wait for the storm to pass by. The reason they started telling people to pull to the middle of the road, was that same year we were there a family pulled to the side and had their car taken away We rarely get enough rain to cause one, and our ground doesn't allow for it.