Delta, a low-lying landform consisting mainly of mud, silt, sand, and gravel laid down by a river at its mouth. As a river enters a large, relatively quiet body of water—such as a bay or a gulf—the flow of the river's water is slowed. As a result, waterborne particles settle to the bottom. Eventually these sediments build up and extend seaward as a landform barely abovesea level. Seaward growth is sometimes rapid—as much as 200 feet (60 m) in a single year in the case of the Po and Mississippi rivers. Though some rivers cross their deltas in one main channel, most divide into branches called distributaries.
A delta is an area of land that forms at the mouth of a river.Deltas vary in shape, depending on the conditions that created them. Some, such as the Nile River delta, are roughly triangular and are called arcuate deltas. It was the Nile delta that led to the name of this landform—it resembled the fourth letter of the Greek alphabet, delta (Δ). Deltas with long fingerlike projections extending far out to sea are known as bird's-foot deltas. An outstanding example is the Mississippi River delta. A cuspate delta, such as the Tiber's, extends into the sea as a V, with long curving sides. An estuarine delta, such as that of the Seine in France, occurs where a river empties into a large estuary.
Because of their fertile soils and location at the mouths of major rivers, many deltas have long been important centers of agriculture and the crossroads of transportation. Among the great cities and ports of the world with delta locations are Alexandria, on the Nile; Calcutta, on the Ganges-Brahmaputra; New Orleans, on the Mississippi; Rotterdam, on the Rhine; and Shanghai, on the Yangtze.
Along some coasts there are no deltas. Their formation is prevented mainly by strong currents and tides; by submergence, or gradual sinking, of the land; or by extremely deep water offshore.
