A claim about raging rivers

We found this display at Maryport aquarium. I was struggling with how to work this out. Once source I found even says the claim is erroneous. I knew I was starting with momentum change but couldn't get the 64 factor. Apparently it is the Sixth Power Law, that mass of object goes with the sixth power of velocity. 2^6 = 64. This source gives a way of getting it: https://physics.stackexchange.com/questions/132620/sixth-power-law