Deltas have long played an important role in human history. These fertile areas where rivers flow into large bodies of water have served as fishing, farming, and living sites. Of the great deltas around the world, perhaps none has had a greater role in civilization than the delta of Egypt’s Nile River. Greek historian Herodotus (c. 484–c. 425 B.C.E.), considered by many as the “Father of History,” studied this great geologic feature. He is credited with coining the term “delta” for this type of landform because its triangular shape reminded him of the Greek letter ∆ (delta).
A delta is a body of sediment deposited at the mouth of a river or stream where it enters an ocean or lake. Unlike other landforms affected by running water, a delta is not created primarily by water cutting into or eroding the landscape (erosion is the gradual wearing away of Earth surfaces through the action of wind and water). Water does not tear down a delta; instead, it builds up a delta.
A river creates a delta by laying down sediment or rock debris such as gravel, sand, silt, and clay that it has picked up and carried along its course. Alluvium (pronounced ah-LOO-vee-em) is the general term for sediment deposited by running water. A river’s depth, its width, and its speed determine how much sediment it can carry. The Mississippi River flows at an average surface speed of about 2 miles (3 kilometers) per hour. Yet it drains between 1.2 and 1.8 million square miles (3.1 and 4.6 million square kilometers), which is more than 40 percent of the total area of the continental United States. Over the course of a year, it moves an average of 159 million tons (144 million metric tons) of sediment.
In general, deltas are similar in shape to another type of landform deposited by flowing water, alluvial (pronounced ah-LOO-vee-al) fans.
Found typically in desert and other arid (dry) environments, these fanlike deposits of sediment form where an intermittent, yet rapidly flowing canyon or mountain stream spills out onto a plain or relatively flat valley. An alluvial fan is a landform that forms on land. A delta is a landform that forms in water.
Ganges River Delta, Bangladesh and India
The combined Ganges and Brahmaputra Rivers have formed the largest delta in the world, the Ganges River Delta (sometimes called the Ganges-Brahmaputra Delta). Approximately 220 miles (345 kilometers) wide, the delta covers an area of roughly 40,790 square miles (105,645 square kilometers). Almost 1.1 billion tons (1 billion metric tons) of sediment is discharged from these rivers annually. This has produced the Bengal Fan, a deposit of sediment on the floor of the Bay of Bengal that stretches outward 1,865 miles (3,000 kilometers) in length and 620 miles (1,000 kilometers) in width. In places close to shore, the Bengal Fan measures up to 7.5 miles (12 kilometers) thick.
A glacier at about 22,100 feet (6,735 meters) in the Himalayan Mountains is the source of the Ganges River. The river flows southeast across India to combine with the Brahmaputra in Bangladesh. The Brahmaputra River has its source in Tibet along the northern slope of the Himalayan Mountains. From there, it flows across the Assam Valley of India into Bangladesh. The rising Himalayan Mountains provide much of the sediment load of these two rivers.
In Bangladesh, the rivers join to form the Padma, the main channel to the sea. The united rivers branch into many distributaries, forming the vast and fertile Ganges River Delta. The delta region covers roughly 25 percent of India’s total territory. The delta’s southern portion is covered largely with a swamp forest roughly 6,525 square miles (16,900 square kilometers) in area. Known as the Sunderbans, it is home to the endangered Royal Bengal tiger.