DELTA

low-lying plain that is composed of stream-borne sediments deposited by a river at its mouth. Some 2,500 years ago, Herodotus recognized that the land bounded by the seaward-diverging distributary branches of the Nile and the sea was deltoid in shape and so used the Greek letter D (delta) to describe it. Although many of the world's deltas are deltoid, or triangular, in shape, notable exceptions exist. In most cases, the delta shape is controlled by the outline of the water body being filled by sediments. For this reason, the term delta is now normally applied, without reference to shape, to the exposed and submerged plain formed 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 display much variation in size, structure, composition, and origin. These differences result from sediment deposition taking place in a wide range of settings. Numerous factors influence the character of a delta, the most important of which are: climatic conditions, geologic setting and sediment sources in the drainage basin, tectonic stability, river slope and flooding characteristics, intensities of depositional and erosional processes, and tidal range and offshore energy conditions. Combinations of these factors and time give rise to the wide variety of modern deltas. The presence of a delta represents the continuing ability of rivers to deposit stream-borne sediments more rapidly than they can be removed by waves and ocean currents. Deltas typically consist of three components. The most landward section is called the upper delta plain, the middle one the lower delta plain, and the third the subaqueous delta, which lies seaward of the shoreline and forms below sea level. That part of the river confined by valley walls, termed the alluvial plain, serves as a conduit through which sediment and water derived from the drainage basin are brought to the sea. At some point downstream the plain broadens out, and most river channels break up into more than one course. This is the apex of the delta and the beginning of the upper delta plain. All of this land lies at an elevation above the effective intrusion of tidal water and is formed entirely by riverine processes. Areas between channels usually support broad freshwater marshes, swamps, or shallow lakes. The lower delta plain is periodically flooded by tidal waters, and landforms result from the interaction of both riverine and marine processes. Areas between the channels show a variety of landforms, ranging from brackish water bays and mangrove swamps to saline tidal flats and beach ridges. The subaqueous delta forms entirely below the level of the sea and commonly constitutes the obvious bulge on the continental shelf seen seaward of many deltas. It serves as the submarine platform across which the exposed delta eventually builds. Variations in the proportions of each deltaic component give rise to the different sizes and slopes of the world's deltas.