Paul Wagner, age 10, of Louisville, Kentucky, for his question:

What is meant by killing a lake?

A lake is an item of geography and none of the earth's surface features last for ever. The mighty mountains have life spans lasting many millions of years. Com¬pared with them, a lake has a very short life span. It may fill up and dry up after a few thousand years or choke to death after just a few centuries. In nature, this is the normal course of events. But the handiwork of mankind often upsets the earth's natural schedules and balances. When we speak of killing a lake, we mean that human activities are hastening it toward its end.

The balancing factors of nature keep a lake supplied with fairly fresh water. The water that is evaporated and run off by drainage streams is replaced by rain and inflowing streams. It supports a balanced community of plants and animals. When strong sewage and chemical wastes are dumped into the lake, this balance is upset. Fishes and other creatures are poisoned or starve because their natural food has perished. The lake becomes a watery desert that cannot support life. Sometimes algae and other durable waterweeds thrive unchecked in the water of a shallow lake and it becomes a polluted swamp. It chokes and dies, maybe thousands of years be¬fore its time.