What creates recirculating currents and turbulent waters that can swamp boats and drown boaters?

A low-head dam creates recirculating currents and turbulent waters that can swamp boats and drown boaters. Low-head dams are dangerous for boater because they have a significant drop off and fast-flowing water, they are difficult to spot from upstream, Dams can deceive even experienced boater, dams hydraulics are unpredictable.