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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(del -tă) (δ)
a lowland in the lower reaches of a river, made up of river sediments and dissected by a more or less ramified network of arms and channels. The name “delta” is derived from the capital letter of the Greek alphabet Δ (delta). Because of its resemblance to the Greek letter, the triangular lowland near the mouth of the Nile River was called a “delta” in antiquity.
Deltas are formed as a result of the complex interaction of river discharge, wave motion, tides, and wind-driven onshore and offshore currents. The formation of a delta where there is a shallow beach begins with the appearance of spits near the mouth and underwater bars (deposits) in the riverbed or bars (estuarine bars, “alluviums”) at the edge of the sea. At high water, the deposits and estuarine bars gradually appear above the water and become low-lying islands that divide the riverbed into arms. Where the coastline is deeper, river-borne sediment is built by the waves into shoreline embankments that border the seaward edge of the delta.
Favorable conditions for the rapid growth of a delta are an abundance of sediment deposited by a river, in some cases the lowering of the level of the waterway (or the tectonic elevation of the bank), the location of the river mouth at the apex of a bay or in a lagoon (obstructed deltas), and the shallowness of the basin into which a river discharges. The formation of deltas is impeded by strong tidal currents, wind-driven onshore and offshore currents, and the rapid elevation of the level of the body of water (rapid tectonic submergence). The rate of growth of deltas varies widely from several meters to hundreds of meters a year. For example, while the level of the Caspian Sea remained stable (1863-1914), the linear growth of the Volga delta was 94 m a year, but between 1930 and 1940, the delta grew by up to 0.7-1 km per year.
Deltas evolving in the apexes of bays are called fill deltas, and those that develop on an open seacoast are called protruding deltas. A number of types of deltas are classified according to the method of formation and the stages of development: cuspate deltas (the Tiber), bird-foot deltas (the Mississippi), obstructed deltas (the Kamchatka), protruding deltas with an open, rounded seaward edge (the Niger), and multi-island deltas (the Volga). When there is a small quantity of sediment and the sea level drops, the river arms dissect the deposits that make up the surface of the estuarine section of land, which is of marine or other origin, and serrated deltas are formed (the Neva).
The deltas of major rivers are often large: the area of the Amazon delta is 100,000 sq km, the Lena delta, 28,500 sq km, and the Volga delta, 19,000 sq km. The fertility of the soils and their moisture make lands of a sizable portion of many deltas very valuable as areas for intensive farming (for example, the deltas of the Nile, the Huang, and the Ganges).
REFERENCESSamoilov, I. V. Ust’ia rek. Moscow, 1952.
Leont’ev, O. K. Osnovy geomorfologii morskikh beregov i dna. Moscow, 1961.
Zalogin, B. S., and N. A. Rodionov. Ust’evye oblasti rek SSSR. Moscow, 1969.
O. K. LEONT’EV
ii. In parachuting, it means a free-fall position where the arms are held in a sweptback position from the shoulder. This position is used for horizontal travel.
iii. An aircraft with delta wing.
delta aircraft An aircraft with a delta (triangular shaped) wing. See delta (iii).
delta hinge The hinge about which the helicopter rotor blades are free to flap in a plane parallel to the hub axis.
1. An expression-based language developed by J.C. Cleaveland in 1978.
2. A string-processing language with single-character commands from Tandem Computers.
3. A language for system specification of simulation execution.
["System Description and the DELTA Language", E. Holback-Hansen et al, DELTA Proj Rep 4, Norweg Comput Ctr, Feb 1977].
4. A COBOL generating language produced by Delta Software Entwicklung GmbH.