ENGINEERS!?!?! You want a physicist!

And if you ignore enough factors, this becomes a really simple problem. The question then becomes: which factors are ignorable? But for now, let's go into a little more detail on that 8 meters estimate.

Let's use conservation of energy (meaning that we will ignore any energy loss on impact and any friction once in the water - pretend that the paddler makes an absolutely perfect oregon tuck to get maximum depth). The paddler's energy just as they enter the water will be mgh where h was the height of the waterfall. The buoyant force (which we'll approximate as constant) will be equal to the weight of the displaced water; rho V g, where V is the volume of the boat and paddler. The total force will be this buoyant force and gravity. Recall that work is the change in energy, and work is the integral of F dot dx. The dot product switches the sign, and when the paddler comes to a stop at the instant of greatest depth d, we will have:

mgh = (rho V g - mg) d

so that

h= (rho V / m - 1) d.

When I used a mass of paddler +boat of 100 kg (since it's a round number), and a volume of .4 m^3 (which is a little over 100 gallons, since it also includes the upper body of the paddler), and recalling that rho = 1000 kg/m^3, I got about 9 meters for h.

Is this the "real" answer? No, of course not. First, the time spent at that depth is very short, which I don't think gives the water time to work its way through the seals. Second, the impact with the surface would steal a lot of the energy that we assumed, meaning that taller waterfalls would be safer. On the other hand, there's not just buoyant force acting on you after you land, there is often water moving down (from that waterfall you just ran) that will pull you deeper than impact alone would. That factor would make the height of the waterfall mostly irrelevant.

I think the only solution is to turn to experiment. Run the brown. Fred Gnorquest had a helmetcam when he ran Metlako a little while back. We should ask him if the seals held.

-Leif ANDERSON