Why are low head dams dangerous to boats?

Low Head dams often have tops that are below the water surface. But aside from their lack of visibility, they can create a fatal trap for small boats. Water going over a low-head dam creates a strong recirculating current (a backroller) and a turbulent upflow (a boil). These currents can push boats back against the downstream side of the dam, where the boats may capsize and their occupants may be pulled under.