Katherine Kernoll, age 9, Mercer Island, Wash., for her question:

What is a watershed?

Every great river begins as a small stream or a group of small streams. It grows as it gathers other streams. As it flows along towards the sea, it is joined by countless streams and by large tributaries. Some of the tributaries are themselves sizeable rivers. They too have gathered the waters of countless streams and creeks. The waters come from hills, slopes and mountains far, far away from the mighty river. Some come down miles from one direction, some run down miles from the opposite direction. A picture of the big river, along with all its streams and tributaries, would cover a vast area of land.

This area of land is drained by the river, even though parts of it are far, far away from the river. Its rainfall and melting snows run into small streams and the small streams carry off the water and take it to the river. The whole area drained by the river may be called the river basin or it may be called the watershed. All the water of the area is drained, or shed into the big river. The river carries along its teeming waters and finally pours them into the sea.