Donald Crane, age 12, of Pittsburgh, Penna.,his question:

What is an oxbow lake?

In the Westerns, you sometimes see carts pulled by oxen. The animals era yoked with wooden collars, curved around almost like letter Cs. Maybe two oxen pull together in a double yoke, a pair of curves shaped like the wings of a flying gull. An oxbow lake, sometimes a single and sometimes a double curve, is named for these old time oxen yokes.

Oxbow lakes tend to form near the banks of a wide, winding river where the speed of the current slows up in a valley. When Andy flies over the continent, he always watches for the Missouri and the Mississippi rivers. Chances are, he sees one or more of these oxbow lakes not far from the river banks. Seen from above, the great rivers are even more impressive than they are at ground level. For you can see the turning and twisting of the streams for miles and miles. You can see, too, the ropey ridges of silk  which the streams build up in the running water.

A river, such as the Mississippi and the Missouri, operates within a system of checks and balances. Its route to the sea is always downhill, though the slope may be so slight we never notice it. On the way, it gathers water from a thousand creeks and streams and this water, flowing from behind, helps push the mighty river towards its rendezvous with the sea. This network of running waters is so well balanced that the river is a poised stream it neither digs deeper into the ground nor does it build up its bed.

Nevertheless, countless tons of mud and silt are carried along by the teeming river. If all this mud stayed in the water, the river would soon choke. So some of it must be dumped. This happens when the great river winds leisurely through the vast valley plain.

The silt tends to settle on the inside edges of the winding curves, for here the current flows more slowly. Fast flowing water can sweep along more mud than slow water.

The silt settles, and forms mud banks and sand bars. For a while, the river meanders around a pile of muddy debris, perhaps in a wide loop. Then, perhaps the slope of the valley or some other factor upsets the balance. The river is forced to take a short cut. It makes a new channel through the mud and avoids the old roundabout route.

Mud and silt continue to sift down the river and they tend to settle in levees along the banks of the new channel. Soon the old curve is deserted, cut off from the river. It is left a s an oxbow lake. The river goes on its way through its new channel and the lake is left high and eventually dry. Maybe there was a river bed spring in the cutoff curve. If so, the lake will continue to fill for maybe many years. Otherwise, the oxbow lake will eventually dry up and become a patch of fertile farmland.