Why do rivers wiggle instead of flowing straight?

Rivers wiggle because every small bend digs itself deeper. When water rounds a curve, it eats away at the outer bank and drops sand and gravel on the inner bank. The outside retreats, the inside grows, and the bend swells into a loop. Repeat that along the whole river and you get a snake-like channel swinging back and forth across its valley. Geographers call each loop a meander.

The secret is a hidden corkscrew. In a bend, the water does not just flow forward: near the riverbed, friction slows it down, and the water there is pushed from the outside of the bend toward the inside, while near the surface it flows back toward the outside. This spiral current sweeps loose material to the inner bank and leaves the outer bank bare and exposed, so the bend erodes even faster. The more a river curves, the more it curves.

You can see the same corkscrew in a cup of tea. Stir it, and the leaves gather in the middle of the bottom instead of flying to the edges. The physicist James Thomson first explained why in 1857, and in 1926 Albert Einstein wrote a paper using the same idea to explain how rivers erode their banks.

The word itself comes from the Maiandros, a famously twisting river in what is now Turkey. The Greek geographer Strabo said it wound so much that everything winding was called meandering.