What is a delta?

   A river almost always carries a load of sand and mud and pebbles. Many rivers carry their loads to the sea. When the water of a river strikes the sea, it is slowed up. In most cases it cannot carry its load far. It drops first the pebbles, then the sand, and then the mud. It may drop so much that a f an of land is built up at the mouth of the river.
   Such a fan of land is called a delta. It got its name because it is a little like the shape of the Greek letter called delta. Delta is the Greek letter from which our D came. It is shaped like this: Δ.
In the Old World one of the most famous deltas is the delta of the Nile. Much of the fertile land of Egypt is in this delta. The delta helped ancient Egypt become one of the cradles of civilization.
   In the United States there is a great delta at the mouth of the Mississippi River. The city of New Orleans is many miles north of this delta. But at one time the land where New Orleans stands was, scientists believe, a part of the Mississippi delta. Over many centuries, the river kept on bringing down millions of tons of mud and sand to it each year. The mouth of the river—and its delta —kept moving farther and farther south. As more years continue to go by, the delta of the Mississippi is almost sure to extend even farther out into the Gulf of Mexico than it does now.
   The Amazon, the Ganges, the Indus, and the Rhine are other rivers that have built up big deltas. Some rivers flow too swiftly to build a delta. The Congo is one.