Debbie Doittiux, age 13, of  A.B., Canada, for her questioni

What causes floods?

Many factors are responsible for water to gather and sweep these mighty downpours over fields and populated places. Some of these disastrous floods occurred because the natural runoff system could not cope with the sudden, excessive rainfall. Some of the damage, let's face it, might have been averted by better man made flood control systems.

Every year the sky sheds about 24,000 cubic miles of water onto the land. The world wide yearly average is 40 inches of precipitation. However, no spot on earth can count on this yearly quota. It varies with the weather. During any season, almost any place may be parched with drought or deluged with floods.

This has been going on since tine began. meantime the earth has used its gravity and the erosion of running water in constructing defenses to cope with the seasonal downpours. A map of any region reveals a natural network of streams, patiently dug to drain the runoff down the slopes to the sea. What the map doesn't show is a natural backup sys¬tem that stores reservoirs of groundwater below the surface.

In unpopulated areas, most rivers are bordered with dense vegetation. The roots of trees and even grasses help to control, floods by absorbing tons of water from the rain soaked soil. Along some mighty rivers, there are lakes that help to contain surplus water during the rainy seasons, But sometimes a series of major storms, or an unexpected hurricane, proves too much for nature's drainage systems. Then an unavoidable flood occurs.

In populated areas, much of the landscape has been remodeled. In the past, earth's natural drainage systems were ignored or even demolished. Vegetation was tripped from along the rive," and stream banks were  eroded. Such a river could no longer carry away the normal rainfall, and every spring the surrounding farms and cities were deluged with floods.

In modern times, land engineers have found ways to construct man made flood controls. Usually the banks of a flood prone river are rebuilt and reinforced with high, durable levees. The banks, the surround¬ing borders and the faraway hills of the drainage system are planted with grasses and other well rooted vegetation. These steps may be enough to contain the average runoff, plus most major storms.

However, some mighty rivers flood every year even when surrounded by miles of forest. These call for major engineering projects. One method is to construct dams along the course of the river so that surplus water is held back and stored in reservoirs. This idea is borrowed from nature's lakes but we make it do more than control floods.

Our man made reservoirs store surplus rains to supply cities and to irrigate surrounding farmlands. And the man made dams are geared to turn turbines that supply electric power for miles and miles around. Someday, perhaps, our busy world will find time to construct all the flood controls needed to contain all the seasonal rains, plus a series of major storms or a sudden, unexpected hurricane.