I saw some (digital) photos of a handful of people amusing themselves in a nice little “canyon”-like gorge in the Kimberley area, rinsing off under a bucket-or-two-a-second of water running down on the back of the gorge.
Next came some photos from 8 minutes after a ver similar scene (same place on a different day). Some gentle rain upstream had enthused the water (in those 8 minutes) to run up from bucketfuls to many tens of tonnes of water per second.
I say (somewhat vaguely) “tens of tonnes” of water because the flow was also heavily silted, which makes any guesstimate of mass kind of inaccurate, but in real life we’re probably seeing around about 20-30 cubic m (≈30-40 tonnes) every second through a runoff lip maybe 10m wide. The cameraman — although positioned by the top rim — was rapidly forced to leave lest the view go a little... well, downhill all of a sudden.
Let’s just leave the summary description at “swimming didn’t look very much like a survival characteristic here any longer”. And that’s anywhere in the gorge-ish at all. Think champion surfing potential down tens of m of narrow, rocky gorge — at maybe a hundred or so km an hour. No pauses.