 Rivers and oxbow lakes, Migrant champ
Q: How are oxbow lakes formed? John, Albuquerque, New Mexico
Mississippi River, meandering north of Vicksburg, Mississippi. The radar image shows an
oxbow lake (shown as black, and looking like an oxbow to right of center) and an oxbow scar (shown greenish yellow,
and looking like a horseshoe about in the center of the image). Satellite
view, image courtesy of JPL/NASA
A: Oxbow lakes happen because Paul Bunyan's giant blue ox, Babe, pulls the rivers straight and leaves
her big hoof prints behind. A tall tale, but oxbow lakes do look like giant horseshoes.
A meandering, winding river can make a bend that is almost a complete loop. When the river cuts through
the neck of the loop, shortening its course, silt builds up across the old channel and blocks water from
flowing in. The cut-off loop becomes a lake--shaped like an oxbow--and the river migrates away from it.
The name oxbow comes from a U-shaped piece of wood that fits under and around the neck of an ox. One-loop lakes resemble oxbows
but, when the river cuts off many loops, the lake appears snakelike. Eventually, an oxbow lake fills up, creating first a marsh and finally a
meander scar on the land.
Oceans have their oxbows, too: circular eddies, called rings. The fast moving Gulf-Stream current twists and curves in great sweeping
meanders after passing Cape Hatteras off the coast of North Carolina. When a meander becomes really sharp, it may pinch off and form a
ring, like an oxbow lake.
Rings form either to the north or to the south of the Gulf Stream. The northerly rings have a warm-water core, warmer than the
surrounding water. They swirl waters up to 250 miles across. The southerly ones have a cold core.
Winding rivers form oxbow lakes because the river cuts off a loop. The banks erode at the neck of the bend and silt fills the inside. The
cause of ocean rings eludes us. Certainly, next to the Gulf Stream oxbows, the same explanation fails: no banks to erode or silt to fill.
What is the reason?
Wait. My shortwave radio is receiving a message from USS Deyo, a destroyer on maneuvers off the coast of Cape Hatteras. Much
static... They've spotted a large creature. A great blue ox? Striding toward Norway pulling the Gulf Stream straight! This can't be!!
Huge eddies are forming where she pulls her hoofs free. Warm- and cold-core rings! The reception is fading. More later...
Further Surfing:
JPL/NASA: Mississippi River image description
Q: What animal migrates the farthest?
A: The Arctic tern wins easily. She follows summer, flying about 22,000 miles (35,000 km) a year as she wanders from the Arctic to the
Antarctic and back again.
Even an attentive bird watcher could confuse an Arctic tern with its mirrored cousin, the Antarctic tern. Some didn't believe that a foot-long (30 cm) bird could fly that far. Then we ringed an Arctic tern in Labrador, released it, and recaptured him in South Africa, 11,000
miles away, three months later. What's more this bird had just learned to fly when let go.
Terns rarely rest on waves along the way and almost never swim. Instead they dive bomb for krill, shrimp, and plankton blooms to eat on
the fly. Up they careen to continue their slow, butterfly-like flight.
Further Reading:
The International Wildlife Encyclopedia, edited by Maurice Burton and Robert Burton.
(Answered Oct. 25, 2002; updated Sep. 11, 2007)
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