Gary McHenry, age 9, Of Sarasota, Fla., for his question:

What is the continental divide?

North America is a continent of dry land. It floats like a table top in the wide oceans. There are valleys and mountains and the continent is not level. The high levels slope down to the seas and the parts of the table top are tipped up or down. Its rivers and streams must flow downhill and follow the slopes of the continent.

They cannot flow uphill over the ridges. Certain ridges are dividing lines that make some rivers take one route and others take an opposite route. The most famous of these dividing lines is the continental divide along the western mountains. Rivers that start on its western side must flaw westward toward the pacific. Rivers that start on its eastern side must flow eastward.