Betty Cook, age 14, of Don Mills, Ontario, Canada, for her question:

Why does the Nile flood every year?

The river that flows through your town is merely a tiny portion of a mighty waterway. The vast river system gathers streams from far and wide and together they merge and drain the water and the overflow from an immense territory. With the changing seasons, the level of your river rises and falls. This is because its supply changes with the ground water conditions throughout the immense territory. In spring when extra rainfall adds to the melting snows, the supply of streaming water may be too much for the river to carry away all at once. It fills to the brim and spills over its banks to flood the surrounding land.

This story is repeated by many great rivers. Every year the Nile floods over its banks soon after the summer rains have drenched tropical jungles far to the south. For thousands of miles along its northward course, the great river is joined by countless streams. The seasonal rainfall throughout the immense area rushes to swell the Nile. As it nears the Mediterranean, it flows through the flat lowlands of Egypt and here the brimming river slows down and floods over its banks.