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.

All of them are man made

Humans can affect weathering and erosion in several ways.  humans cause  increases in acid rain and pollution, which increase the amount of  weathering  agents in the air and water, and then on land.for an easy answer: 1) acid rain2) pollution