delta

An arcuate delta and a bird's foot delta. The Mississippi delta is an example of a bird's foot delta and the Ganges delta is an example of an arcuate delta.

River sediments deposited when a river flows into a standing body of water with no strong currents, such as a lake, lagoon, sea, or ocean. A delta is the result of fluvial and marine processes. Deposition is enhanced when water is saline because salty water causes small clay particles to adhere together. Other factors influencing deposition include the type of sediment, local geology, sea-level changes, plant growth, and human impact. Some examples of large deltas are those of the Mississippi, Ganges and Brahmaputra, Rhône, Po, Danube, and Nile rivers. The shape of the Nile delta is like the Greek letter delta or Δ, and gave rise to the name.

The arcuate (arc-shaped) delta of the Nile, found in areas where longshore drift keeps the seaward edge of the delta trimmed and relatively smooth, is only one form of delta. Others include birdfoot (projecting) deltas, where the river brings down enormous amounts of fine silt, as at the Mississippi delta; and tidal (linear) deltas, like that of the Mekong, China, where most of the material is swept to one side by sea currents. Cuspate deltas are pointed, and are shaped by regular opposing, gentle water movement, as seen at the Ebro and Tiber deltas in Italy.

The material deposited as a delta can be divided into three types.

Bottomset beds The lower parts of the delta are built outwards along the sea floor by turbidity currents, which are loaded with material. These bottomset beds are composed of very fine material.

Foreset beds Inclined, sloping layers of coarse material are deposited over the bottomset beds. Each bed is deposited above and in front of the previous one, the material moving by rolling and saltation. The delta thus builds up in a seaward direction.

Topset beds These are made up of fine material and are a continuation of the river's flood plain. They are extended and built up by the work of numerous distributaries formed when the main river splits into several smaller channels.

Delta

Town in Greater Vancouver Regional District, southwest British Columbia, Canada; population (1991) 89,000. It is located on a peninsula between the Fraser River to the north and Boundary Bay to the south, 19 km/12 mi south-southeast of Vancouver city centre. A residential suburb expanding into an agricultural area, it has truck gardens and dairy farms. Industries include fish canning, food processing, and paper milling. Ferries to Vancouver Island, the Gulf Islands, and other points leave from Tsawwassen terminal.

Delta

Town in Millard County, west-central Utah; population (2000) 3,200. It is situated 48 km/30 mi northwest of Fillmore and 167 km/104 mi south-southwest of Salt Lake City, near the Sevier River in the Pahvant Valley, on the east edge of the Great Basin. From 1905 it was developed as an irrigated farming centre, producing alfalfa seed. A large beryllium processing plant is here. Topaz, 24 km/15 mi to the northwest, is the site of a camp where over 8,000 Americans of Japanese descent were interned in 1942–45.