Bathtubs and Lake Peigneur
A whirlpool is a vortex, a spinning body of water around a central point. Whirlpools can vary from tiny eddy currents in a stream to huge tidal flows powerful enough to swamp small boats. Most of us have our first experience with whirlpools looking at water going down the sink, however this is not how most whirlpools in nature work. When the drain is opened, all of the water tries to flow down the sink at the same time, while air in the drain tries to push up to equalize the pressure. The water begins to spin and form into a stable vortex--the water flows down, spinning along the sides of the vortex while air flows back up through the center. One notable large whirlpool worked in just this fashion. When oil drillers in Lake Peigneur in Louisiana accidentally drilled into a salt mine, the water started to flow down into the earth, causing a huge vortex. This vortex was far more destructive than most natural whirlpools, flooding the mine, swallowing barges, and even sinking the entire oil platform.
Tidal Whirlpools
Tidal whirlpools are much more common in nature than the type described above. A tidal whirlpool occurs when two currents are moving in opposite directions, or at right angles to each other. Friction between the two currents causes the water to spin, creating the whirlpool. To get a better idea of why this happens, imagine running towards a friend so that you pass right past each other and linking hands as you pass. This causes you to spin around in a circle, since you still are moving but can't continue in your original direction. The water currents don't literally "grab" each other, but they do rub against each other as they pass, creating drag and causing them to spin.
Other Whirlpools
Whirlpools can form anywhere where the water flow is turbulent. Small whirlpools called eddies, for example, often form near the edges of fast-flowing rivers. The water in the middle of the river flows quickly, while the water at the edges tends to pool and flow more slowly, causing small vortexes to form where the two meet. Whirlpools can also form under waterfalls. The fallen water hits the still lake or basin beneath the falls, creating turbulence. If the waterfall is large enough, this can cause quite powerful whirlpools.
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