Low head dams, spillways, and other manufactured water impediments create among most dangerous recirculation spots regardless of river classification, and so should be fully avoided at all costs. And irrigation ditches too can be real killers. As many have said, countless have drowned this way, lowly tubers, strong boaters, sadly also a lot of children.
Basically, the laminar flow or "sheet" of water that a pour-over creates becomes a strong, vertical hydraulic force of displacement entering the pool below, trapping things or pulling them back under the falls by the recirculation. Since this can entrain debris, being knocked out by this stuff is possible as well. So in short, ABSOLUTELY DO NOT TRY IT. Sure, there's a lot of videos of death-defying morons doing things and they miraculously aren't hurt/maimed/crippled/dead. But don't let exceptions become the rule to try stuff with a significant penalty of death because YMMV.