phenomenon 

Whirlpool

                  A whirlpool is a body of rotating water produced by opposing currents or a current running into an obstacle . Small whirlpools form when a bath or a sink is draining. More powerful ones in seas or oceans may be termed maelstroms. Vortex is the proper term for a whirlpool that has a downdraft.

             In narrow ocean straits with fast flowing water, whirlpools are often caused by tides. Many stories tell of ships being sucked into a maelstrom, although only smaller craft are actually in danger. Smaller whirlpools appear at river rafid  and can be observed downstream of manmade structures such as weirs and dams. Large cataracs, such as niagara fails , produce strong whirlpools.

Whirlpools in the ocean are usually caused by tides. Very small whirlpools can easily be seen when taking a shower or when draining a sink, but this vortex is produced in a way that is very different from what is in nature. A small whirlpool also appears at the base of many waterfalls. In the case of strong waterfalls, such as Niagara Falls, this whirlpool can be very strong. The most powerful whirlpool known today is a whirlpool that forms in a shallow narrow strait, with fast flowing water.

                                  When river water is forced to rotate around objects or to flow into narrow rivers, water flows faster and is more likely to create energetic rotating turbulence. At sea, depending on the geology of the seafloor, ocean currents can collide and create conflicting tidal currents. Water rotates counterclockwise north of the equator and south of the equator in a clockwise direction. Its destructive interaction can form a whirlpool, a strong rotating water current. A Maelstrom, a swirl of strong water currents that have vortex, is the deadliest of all.