Hundreds of years ago, Greek merchants who sailed to the ports of Egypt noticed that the Nile River branched into two streams before flowing into the sea. The two branches of the river and the sea enclosed a piece of land in the shape of a triangle. They named that section of land a delta because it resembled the fourth letter of the Greek alphabet (Δ). Eventually the term came to be used for the land of any shape that is at the mouth of a river. A delta consists of the sediment that is deposited by a river as it empties into a lake or sea. As a delta grows, the flow of the river is hampered due to the deposits. As a result the river splits into a number of smaller branches known as distributaries before it reaches the sea. Deltas vary in size, shape, composition, and origin. Well-known deltas of the worldThe Ganges and Brahmaputra rivers of South Asia have created the largest delta in the world. This delta is more than 200 miles (320 kilometers) wide. The delta of the Mississippi River in North America stretches out 200 miles (320 kilometers) into the Gulf of Mexico. It is still growing at the rate of 1 mile every 32 years (1 kilometer every 20 years). The importance of deltasThe soil that is carried by a river to form a delta is usually very fertile and ideal for farming. For this reason a number of deltas are inhabited, and people continue to live on them despite the danger of floods. The huge delta of the Mekong River in Vietnam is known as the rice bowl of Southeast Asia because of the enormous amount of rice grown there. In recent years it has been discovered that large amounts of the world's petroleum (oil) resources can be found in ancient rocks under some deltas. |