Dorothy Kanelakos, age 13, of Arkansas City, Kansas, for her question:

What makes a river take a winding course?

The course of a river is always downhill, even when it seems to be flowing through a flat valley or over a tabletop plain. Valleys and plains are parts of continents and these larger land masses are always tipped slightly in one direction or another. This continental tilt affects the general course of a river but many local factors cause it to wind this way and that.

The fastest, and usually the straightest, rivers are those which flow down steep hillsides. The lazy winding rivers are those which flow through flat, muddy plains. The Mississippi is such a lazy river and soon from the air it seems to be almost as twisted as a pretzel. Sometimes a side winding loop almost touches the side winding loop ahead of it. Each bend is caused by banks of silt dropped at the edges of the swirling current. These banks force the stream to flow around them.

Sometimes the Mississippi bites a new channel through the mud, a shortcut from one loop to the next. A bend in the river is then left behind to dry up as an ox bow lake