I heard it described as follows: the full force of the river you see upstream has been turned on its side and the current goes down. You can’t pull against that, especially against curved, mossy, wet rocks
So as if an elephant were pulling you down beneath the water and you only had your strength to grip against it. Can’t be done.
So parallels can be made to other places to show the danger. It's a slot canyon; only one that's generally constantly full versus empty most the year until the monsoon hits like those here in the US where I am. But the danger remains the same, the sandstone geology allows for it to create very deep and fast flowing situation due to Bernoulli's principle. Even more so for constrictions in the rock which form natural venturi.
<s>I looked but was not able to find CFM/CMM for the input and output</s>; but that would give a reasonable idea of what the volume of the canyon must be because what goes in must come out. So the volume of the flow at the outlet and inlet gives a very good idea of just how crazy it can be.
What I don't understand is how that works from a conservation of mass perspective. The water can't be literally flowing down at every point and keep a stable surface. I assume it's something more complicated, right.
In an incompressible flow (e.g. water under anything resembling normal circumstances), there is no net flow into or out of any given volume, i.e. a 3-D box or other shape. But the surface is 2-D! So you can look at a little box with its top at the surface and its bottom slightly below the surface. If there is downward flow out the bottom (and zero flow out the top, of course), there must be net inward flow in the sides. Nothing wrong with that.
This could happen if the river has constant speed and constant cross-sectional area everywhere but gets progressively narrower and deeper. The water moves inward because it has to get away from the encroaching banks and moves down to fill the deepening channel. If this is abrupt enough even in the absence of turbulence, the downward current a foot or so below the surface will be substantial.
Part of the difficulty of turbulent water is that it can really whip you and smash you around. It doesn’t necessarily look that violent from outside of the water, but you could be getting broken limbs and brain trauma in that kind of turbulence. The force of the water pressing you into walls and boulders is tremendous.
Loads of fatalities in turbulent water occur because the victim rapidly becomes unconscious in the water.
In addition to the explanations of the turbulent water smashing victims against the rock walls, presumably a very turbulent flow could also turn into a mixture of air and water, with a corresponding decrease in density such that it would be more difficult or even impossible for a person to maintain buoyancy.
It's an underground tunnel with a small exposed strip at the top.
the water can be circulating around the tunnel, flowing back and forth under the rocks. It can be pulling sideways or down at the exposed strip with the water circulating back up in the underground areas out at the sides.
I bet it's flowing horizontally but faster the deeper you go. You fall in, the higher speed flow at your feet apparently "sucks" you under due to its higher speed (higher inertia), like being grabbed by your ankles and yanked down.
Of course the flux of water into any volume is equal to the outward flux. But I would imagine the situation is that there are circulating turbulent branching streams of current pulling you down. If you were incredibly lucky, and not killed by impacting the walls, you might pop back up by catching a ride on another section of current.
So as if an elephant were pulling you down beneath the water and you only had your strength to grip against it. Can’t be done.