Low Head Dams

TIL: Low head dams are some of the most dangerous due to the hydraulic jump/boil that they can create, combined with how stealth they can be.  Often subtle raises in a river, and super calm up to the overflow.

Essentially, there is a current created that keeps bringing water back towards the overflow, leading to debris (and people) potentially being trapped in the churning backflow of water, unable to break free with ease.