Deborah Anderson, age 11, of Santa Maria, California, for her question:

What is meant by a valuable watershed?

Geologists use the word watershed to name two different land formations, and both are associated with water. The more common use refers to the parting of water along a ridge of high ground, this can be long enough to span most of a continent. Another name for such a ridge is a divide. Rainc and snows falling on the divide are separated to flow down the slopes in opposite directions. Countless little streams are started on their way. Such a watershed is valuable because it shares the water from the high ground with two vast land areas, instead of letting it all gush down one side of the dividing slope.

Sometimes the word watershed is used to describe the entire basin of a large river system. This watershed is valuable because its streams and tributaries bring water to miles of thirsty land and also drain surplus water during the rainy seasons.