2 minute read

Alluvial Systems

Deltas

Deltas are formed at the mouths of streams that flow into lakes or oceans. They are fan-like deposits similar to alluvial fans, but located in the water rather than on dry land. Like alluvial fans, coarse sediments are deposited close to shore and fine-grained sediment is carried farther out to sea. The Mississippi River has formed the most prominent example of a delta within the United States. Other well-known examples are the Nile Delta of North Africa and the Amazon Delta of South America. When Aristotle observed the Nile Delta, he recognized it was shaped like the Greek letter delta, hence the name. Most deltas clog their channels with sediment and so must eventually abandon them. If the river then flows to the sea along a significantly different path, the delta will be abandoned and a new delta lobe will form. This process, known as delta switching, helps build the coastline outward.

Resources

Books

Knighton, D. Fluvial Forms and Processes. London: Arnold, 1998.

Leopold, L. B. A View of the River. Cambridge, MA: Harvard University Press, 1994.

Leopold, L. B., M. G. Wolman, and J. P. Miller. Fluvial Processes in Geomorphology. New York: Dover Publications, 1995.

Periodicals

Bull, W. B. "Alluvial fans." Journal of Geological Education 26, no. 3 (1978): 101–06.

Other

Ableman, C. "Glossary of Hydrologic Terms." 2000 [cited October 19, 2002]. <www.srh.noaa.gov/wgrfc/glossary/>.

Pidwirny, M. J. "Fluvial Landforms." 2000 [cited October 19, 2002]. <www.geog.ouc.bc.ca/physgeog/contents/11j.html>.

William C. Haneberg

KEY TERMS

Bajada: —A feature produced when adjacent alluvial fans overlap or coalesce and form a continuous deposit at the foot of a mountain range. Bajadas are common features in arid to semi-arid regions of the American West and Southwest.
Base level: —The lowest elevation to which a fluvial system grades or adjusts itself. Local base levels can be lakes or larger rivers. The global base level is sea level, which changes through geologic time.
Bedload: —The portion of sediment that is transported by rolling, skipping, and hopping along the stream bed at any given time because it is too heavy to be lifted by flowing stream water. It stands in contrast to suspended load.
Discharge: —The volume of water flowing across an imaginary vertical plane perpendicular to the stream channel per unit of time. In the United States it is customary to express stream discharge in units of cubic feet per second.
Gradient: —The slope of a stream channel, measured in terms of the change in elevation per unit of channel length. Stream gradients can be expressed as percentages or using dimensionless terms such as meters of elevation change per kilometer of channel length.
Sinuosity: —The degree of curvature of a stream channel as viewed from above. Highly sinuous streams contain many curves or meanders along the lengths.
Suspended load: —Sediment particles transported within flowing stream water at any given time, as opposed to bedload. Suspended load is responsible for the muddiness or turbidity of river water.

Additional topics

Science EncyclopediaScience & Philosophy: Adrenoceptor (adrenoreceptor; adrenergic receptor) to AmbientAlluvial Systems - Alluvium, Commmon Components, Coastal Alluvial Plains, Alluvial Fans, Deltas