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A delta is a deposit of sediment accumulating at the mouth of a river. Byproducts of continental EROSION, deltas form when rivers flow into standing bodies of water (eg, lakes, oceans) rapidly depositing their load of alluvial material (eg, gravel, sand, silt and clay). Deposition results from a loss of transporting capacity caused by deceleration of the current. Deltas vary enormously in scale and complexity, ranging from small lacustrine deltas with lobate shape and tripartite structure (topset, foreset and bottomset) to large, composite birdfoot deltas formed along ocean shorelines.
The shape of large deltas, many of which have persisted over long periods of time, is controlled by stream HYDROLOGY, the nature of transported sediment, wave and tidal dynamics, sea-level changes, crustal movements and basin and coastline configuration. Depending on density differences between the sediment-laden river water and the lake or ocean water, the inflowing water may move down the prodelta slope as a gravity underflow (ie, below the surface), spread out over the surface of the standing body of water, or otherwise mix with it as an interflow. The kind of movement has a bearing on the mechanism of growth. Strong tidal currents and high wave energy (see OCEAN WAVE ENERGY, which tend to disperse sediment, militate against delta formation. Large deltas (eg, Mississippi) are traversed by a network of distributary channels, and delta growth is accompanied by development of a low-lying delta plain associated with different depositional environments, eg, distributary, levee, backswamp and marsh (see SWAMP, MARSH AND BOG). Superposition of depositional environments over time may produce a favourable habitat for oil and gas accumulation, as seen in the MACKENZIE RIVER delta region, NWT. Because of their rich soils, water supply and transportation access, deltas in temperate and tropical climates have played an important role in the history of civilization. However, FLOOD control, land reclamation and channel maintenance have posed problems for inhabitants. In Canada the Mackenzie and Fraser river deltas are outstanding examples of this RIVER LANDFORM.
 Mackenzie Delta, Satellite ImageThis satellite image shows one of the world's largest river deltas. It has formed over thousands of years as the river deposits silt and sand into the Beaufort Sea. The maze of islands and streams are home to wildlife (courtesy Canada Centre for Remote Sensing).
 Mackenzie Delta, Map
 Peace-Athabasca Delta, Satellite ImageImages from RADARSTAT in May 1996 show the Peace-Athabasca Delta, one of the world's largest inland freshwater deltas that covers approximately 3900 square kilometers (courtesy Canadian Space Agency).
Author
ALAN V. JOPLING
Links to Other Sites
Peace-Athabasca Delta, Alberta - Ramsar Site
Brief notes about waterfowl habitats in the Peace-Athabasca Delta region that have been classified as “Ramsar Sites: Wetlands of International Importance.” Check this site for more about the “Ramsar Sites” designation. From the Prairie and Northern Region Green Lane, Environment Canada.
Quenching the Peace Athabasca Delta
This site provides an overview of programs designed to mitigate the impacts of human economic activity and natural variations in climate on the natural productivity and biological diversity found in the Peace Athabasca Delta region of northern Alberta. From Environment Canada.
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