Delta

Deltas are complex depositional landforms that develop at the mouths of rivers. They are composed of sediment that is deposited as a river enters a standing body of water and loses forward momentum. Famous deltas include the Mississippi delta in Louisiana and the Nile delta in Egypt.

Every river flows, under the force of gravity, from its headwaters to its mouth. The mouth of a river is the location at which the river enters a standing body of water, such as a lake, sea, or the ocean. As the river enters standing water and the current is no longer confined to a channel, it spreads out, slows down, and eventually stops. The reduction in speed of the current causes the river to become unable to continue carrying suspended sediment. As sediment is deposited a series of smaller channels, called distributary channels, forms causing the shoreline to build out, or prograde. The landform created is the delta. In smaller rivers with weaker currents, forward momentum may cease almost immediately upon reaching the lake or ocean. This is especially true where the river empties into an area of strong wave action. In this case, no significant delta will be formed. Larger rivers, such as the Amazon, may be able to maintain some current for several miles out to sea, creating an extensive delta.

As a river reaches and enters a standing body of water, sediment is deposited according to grain size. The coarsest sediment, such as sand, is dropped first, closest to the mouth of the river. With progressive distance from the mouth, finer sediment including fine sand, silt, and clay is deposited. This results in a distinct sequence of layers, known as topsets, foresets, and bottomsets. The topsets, as the name implies, are the uppermost layer. They are comprised of the coarse sediment forming the area of the delta that is above sea level. The foresets include fine sand grading into silt and clay deposited in seaward sloping layers beyond the mouth. Bottomsets are made up of clay particles, carried furthest out to sea where they settle into horizontal layers. Although this sequence is

deposited laterally with increasing distance from land, as a delta progrades the bottomsets are covered by new foresets, which are then covered by topsets as sediment builds up, and so on. The resultant coarsening up sequence is a distinguishing feature of deltaic deposits.

The sequence of topsets, foresets, and bottomsets provides an accurate picture of a simple delta system. Large marine deltas are often more complex, depending on whether the river, wave action, or the tides play the most important role. In stream-dominated deltas, fluvial deposition processes remain strongest, and distributary channels build far out to sea. These deltas are known as bird's foot deltas because of the appearance of the collection of channels extending into the sea. The Mississippi delta is probably the most famous example of a bird's foot delta. In wave-dominated deltas, distributary channels are not maintained for any great distance out to sea; rather, wave action reforms their sediment into barrier islands oriented perpendicular to the direction of flow. This type of delta is more compact, and shaped like a triangle. The Nile delta in Egypt is an example of a wave-dominated delta. Lastly, tide-dominated deltas are also compact, but broad tidal channels and sand bars form parallel to the tide direction. The Mekong delta in Vietnam is an example of a tide-dominated delta.

See also Alluvial system; Estuary; Landforms; Sedimentation