Paddling a canoe and draining the bathtub both produce that familiar, spinning hole called a “whirlpool” — wide at the top, narrow down below.
The shape of a whirlpool comes from two forces acting against each other. First, there’s the spinning of the water, which creates a force away from the center of the whirlpool. You can feel the same kind of force by swinging a ball in a circle on the end of a rope. In the case of the ball, the rope provides the second force, pulling the ball back into the center and keeping it from flying off in a straight line.
In a whirlpool there’s no rope pulling the water back to the center. Instead the opposing force is created by the pressure of the surrounding water. So while the momentum of the spinning water acts as a force sending the water off in all directions, the surrounding water pressure forces it back to the center. The faster the water spins, the wider the whirlpool.
Water pressure doesn’t change from one lake or river to the next, but the pressure is always greater under the water than it is on the surface. And that’s why whirlpools are widest at the top.
A better analogy, then, might be a ball swinging around on a long spring instead of a rope. On a weak spring, the ball would make a very wide circle because the spring exerts only a weak force. That’s like the top part of the whirlpool where the water pressure is also very light. Down below, the pressure acts more like a strong spring, and so the circle is a lot tighter.
Beneath the whirlpool, the water still spins but the water pressure is too great to allow any air space at all.