A whirlpool is a body of rotating water caused by opposing currents or a current running into an obstacle. The rotating current forms when a river meets the sea or when fast-flowing water passes through narrow ocean straits.
When a river flows into the sea or another body of water, it brings with it a stream of water moving at a certain speed and direction. The water in the sea or a larger body of water may be flowing in a different direction or at a different speed. When these two bodies of water meet, especially if there are obstructions or changes in depth, it can create swirling motions and eddies. Examples are Saltstraumen Whirlpool (Norway) and Corryvreckan Whirlpool in Scotland.