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, Albuquerque, NM
Meandering goosenecks of the San Juan River near Mexican Hat,
Utah. Photo courtesy of Wikipedia.
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 meandering stretch showing sand and gravel deposits (red) along the inner
bends and erosion (blue) along the outer bends. Drawing courtesy of M. Morisawa,
modified by author.
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.
 A
river cross section showing water dropping at the outside of the bend.
Drawing from A Primer on Water by Luna Leopold and modified by the author.
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. (See figure.)
Rivers meander because it's their nature — just as it's
our nature to wonder why.
(Answered Dec. 1997; updated Aug. 22, 2007)
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