And now A Moment of Science about one of nature's most basic and subtle shapes -- the winding, back-and-forth S-curve of a meandering river.
It's a peculiar kind of curve. You can spot it easily on a map anywhere it appears -- for example, along the Mississippi River between Arkansas and Tennessee. The curve is not a series of pieces of a circle connected together. And it's not one of the familiar curves used in science and engineering to describe waves. It's something different.
It comes about because a river, in a manner of speaking, doesn't like to make sharp turns. A sharp turn offers resistance to the flow of the water.
A river seeks the path that distributes that resistance to flow as evenly as possible along the length of the river. So the path is not jagged like the letter 'W', but curved in a particular way something like the letter 'S'.
You can see that curve elsewhere -- for instance, in the hem of a pleated curtain. The pleats at the top of the curtain try to bend the hem at the bottom into some kind of zigzag, but the fabric resists bending and makes a smooth, even curve like the curve of a meandering river. The ruffles on a ruffled blouse, and loops of a ribbon bow make versions of that meandering-river curve.
What all these examples have in common is that whatever it is that's being bent offers a uniform resistance to bending -- and that determines the final shape.
Of course a lot has been left out of our description of rivers -- for instance, why rivers meander at all -- why they don't take the shortest downhill path. But that's another story.
