Robin D. McSwain, age 10, of King's Mountain, North Carolina, for his question:

Who built the first dams?

His name was Mr. B. Beaver and he started building dams ages before human beings got the idea. He was, of course, a very special furry mammal who lived around inland waterways. The beaver and his busy relatives have been building dams for countless generations. They do so to control the water supplies in their favorite streams and personal ponds. Thousands of years ago, when the last ice age glaciers retreated, countless North American beavers toiled to control the melting waters. Geologists tell us that their numerous dams, tunnels and canals helped to construct the Great Lakes.

Mankind, it seems, first thought of building dams about 5,000 years ago. Settlers along the Nile and the Tigris River of Mesopotamia built dams of clay and stones to control floods and to irrigate their farmlands. Some of these ancient earthworks have been found in ruins. About 2,000 years ago, the Romans built sturdy brick dams throughout their far flung empire. Some of their durable dams are still being used in Italy, Spain and North Africa.