What causes a whirlpool?

Water, we all know, flows downhill   but this is only one of many laws which govern a moving stream. The pushing energy of the running water is twisted by rocks in the stream bed and by uneven edges along the banks. This forms small, swirling eddies and a whirlpool is really a swirling eddy of water on a large scale, It may be caused by water swirling around a deep hole in the stream bed,

The wildest whirlpools occur off shore where deep holes have been churned up in the sea bed, Here the tidal currents add extra energy to the swirling water. These treacherous ocean whirlpools may be calm when the tide is out, but the incoming tide turns them into pits of raging fury,