delta [from triangular shape of the Nile delta, like the Greek letter delta], a deposit of clay, silt, and sand formed at the mouth of a river where the stream loses velocity and drops part of its sediment load. No delta is formed if the coast is sinking or if there is an ocean or tidal current strong enough to prevent sediment deposition. Coarse particles settle first, with fine clays last and found at the outer regions of the delta. The three main varieties of deltas are the arcuate (the Nile), the bird's-foot (the Mississippi), and the cuspate (the Tiber). The Nile, Mississippi, Niger, Rhine, Danube, Kuban, Volga, Amu Darya, Indus, Ganges-Brahmaputra, Ayeyarwady, Tigris and Euphrates, and Huang He (Yellow) rivers are among those that have formed large deltas, many of which are fertile lands that support dense agricultural populations.

Delta Plan, flood control and reclamation project, S Netherlands, in the Rhine River delta. Built in 1957-81, it involved construction of four major dikes (up to 131 ft/40 m high) across the Rhine's four estuaries on the North Sea, three auxiliary dams, and a storm-tide barrage 38 ft (11.5 m) high and 262 ft (80 m) wide across the IJssel River. The project shortened the Dutch coastline by c.440 mi (700 km), reclaimed 6,100 acres (15,000 hectares), and created a freshwater lake (33 sq mi/85 sq km). Two navigable waterways to Antwerp and to Rotterdam and Europoort were left open.

Low-lying plain composed of stream-borne sediments deposited by a river at its mouth. Deltas have been important to humankind since prehistoric times. Sands, silts, and clays deposited by floodwaters were extremely productive agriculturally; and major civilizations flourished in the deltaic plains of the Nile and Tigris-Euphrates rivers. In recent years geologists have discovered that much of the world's petroleum resources are found in ancient deltaic rocks. Deltas vary widely in size, structure, composition, and origin, though many are triangular (the shape of the Greek letter delta).

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