Why Do Rivers Follow Lazy Loops and Bends?

Q: Why do rivers, like the Rio Grande and especially the Goosenecks of the San Juan River, meander?
Eloy Marquis, Albuquerque, NM

A: Why are rivers crooked? Even Einstein and Leonardo da Vinci wondered. Streams and rivers follow winding paths downhill. Pure, icy rivulets snake across glacial ice. The Gulf Stream flows north along the coast from Florida to North Carolina and then meanders in great sweeping curves northeast across the Atlantic.

You almost never see a straight stretch of river longer than ten times its width. The San Juan, more meandering than most, is 6 miles long through the Goosenecks but only travels a distance of 1.5 miles. That's crooked. Even where banks are straight, the deepest part of a river wanders from side to side. All this suggests meandering is an intrinsic property of streams.

Meanders usually appear wherever a river goes down a gentle slope, flowing around obstructions, through fine-grained soil that easily erodes but sticks together well enough to make firm banks. Apparently the Goosenecks of the San Juan formed eons ago on such a flood plain. Then the land uplifted while the stream cut down to shape the 1500-foot chasm the Goosenecks course through now.

A river bends as it adjusts to disturbances, such as, increases in water volume or obstacles that deflects its current. The diverted current follows a new path, bumps into a bank, encounters bank resistance, and erodes the bank--eventually carving a bend. The greater the curve, the faster the water rounds the bend, takes off on a tangent across the river, collides against the opposite bank, and starts carving another bend. (See figure.) This pattern repeats over and over as the current bounces off the banks on its way downstream--creating swings in the river almost as regular as a clock's pendulum.

Curves enlarge because of water dropping to the riverbed. At the outside of the bend, water drops down and moves toward the center--like tea leaves as you stir tea in a teacup. The dropping water deepens the channel on the outside of the bend. The silt-laden water moves across the riverbed toward the inner bend and drops sediment in the slower-moving water there. This forms sand and gravel bars.

Rivers meander because it's their nature — just as it's our nature to wonder why.

(Updated Aug. 22, 2007)