River island
A river island is any exposed landmass surrounded by river water. Properly defined, it excludes shoals between seasonally varying flows and may exclude semi-coastal islands in river deltas such as Marajó.
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These islands result from changes in the course of a river. Such changes may be caused by interactions with a tributary, or by the opposing fluvial actions of deposition and/or erosion that form a natural cut and meander. Nascent vegetation-free shoals and mudflats may dissipate and shift or build up into such islands through deposition; the process may be assisted through artificial reinforcement or natural factors, such as reeds, palms, evergreen trees or willows, that act as obstacles or erosion barriers, so that water flows around them. Islands may be small or large, covering many square kilometers, examples of which are given below.
Regional nomenclature
The term "towhead" implies an islet (small island) or shoal within a river (most often the Mississippi River) having a grouping or thicket of trees, and is often used in the Midwestern United States. Many rivers, if wide enough, can house considerably large islands. The term "towhead" was popularised by Mark Twain's Adventures of Huckleberry Finn.
In England, a river island in the Thames is referred to as an "ait" (or "eyot").
Largest and smallest
Majuli (a non-coastal landmass between two banks of a river), located in the Brahmaputra River in Assam, India, is recognised by Guinness World Records as the world's largest inhabited riverine island, at 880 square kilometres (340 sq mi).[1]
The Encyclopædia Britannica cites another large non-coastal landmass, Bananal Island (an island that divides the Araguaia River into two branches over a 320 km (200-mile) length of water), located in Tocantins, central Brazil, to be the world's largest river island instead, at 19,162 square kilometres (7,398 sq mi).[2]
However, Bananal Island is not considered a riverine island by some geologists, as they consider the Araguaia River to form two distributaries, and Bananal Island to be the landmass between these two distributed rivers. However, Bananal Island is technically an island as it does not touch the main landmass at any point.
Umananda Island, at 0.02 square kilometres (0.0077 sq mi), is among contenders as the smallest permanently-inhabited river island, or islet, with fixed dwellings. Umananda also lies in the Brahmaputra River. Many as tiny as Umananda or smaller, inhabited, exist on the Amazon basin and Bangladesh. Another island of comparable size to Umananda, Hatfield Island in the Guyandotte River in the U.S. state of West Virginia, has no permanent population, but contains several permanent buildings, namely the K–12 schools serving the city of Logan and its surrounding area plus the main branch of the Logan County public library.[3]
On canalised rivers, such as the Thames and the Seine, one-home islands exist, containing houses constructed of permanent materials. Canals reduce erosion of the islands and in particular limit the height of flash flooding by maintaining substantial "heads" of water through barrages. One-home islands improved by river canals include Monkey, Friday, Holm and D'Oyly Carte islands.
Lists of river islands
River islands by area
- Note: Includes some river islands that also have an ocean coast.
Most populous river islands
This list ranks river islands with a population of at least 25,000.
See also
- Ait – Islands found on the River Thames and its tributaries in England
- Anabranch – A section of a river or stream that diverts from the main channel and rejoins it downstream.
- Archipelago – Collection of islands
- Distributary – Stream branching off from main stream channel
- Fluvial landforms – Processes associated with rivers and streams
- Island – Piece of subcontinental land completely surrounded by water
- Lake island – Island located in a lake
- Point bar – Landform related to streams and rivers
- River bifurcation – The forking of a river into its distributaries
- River delta – Silt deposition landform at the mouth of a river
Notes
- Due to redirecting of water from the Rangitata River, the river's southern distributary channel is now dry except after heavy rain or snow melt. As such, Rangitata Island is usually connected to the South Island mainland.
- The listed population is that of the New York City borough of Manhattan. Almost all of the borough's population lives on the island of Manhattan, but the borough also includes the following permanently populated areas:
- Marble Hill, a neighborhood with a population of roughly 10,000 that was originally part of Manhattan Island, but was physically separated from the rest of the island by the opening of a canal in 1894 and in turn physically connected to the US mainland by landfill in 1914.
- Randalls and Wards Islands, the collective name for three conjoined islands with about 1,600 residents, all either patients or staff of psychiatric hospitals, or occupants of homeless shelters.
- Roosevelt Island, located in the East River between Manhattan Island and Queens, with a population of nearly 12,000.
References
- Majuli, River Island. "Largest river island". Guinness World Records. Retrieved 6 September 2016.
- Ilha do Bananal, River Island. "Bananal Island". Britannica. Retrieved 5 August 2020.
- Spence, Robert Y. (February 21, 2012). "Hatfield Island". The West Virginia Encyclopedia. Retrieved June 12, 2020.
- "Río Grande de la Magdalena - Colección Ecológica del Banco". Retrieved 3 January 2015.
- "Žitný ostrov". Retrieved 3 January 2015.
- "崇明县". www.shanghai.gov.cn.
- "Largest Islands of Countries". Retrieved 3 January 2015.
- "Malebo Pool". Encyclopædia Britannica. Accessed June 2017.
- "SUMMARY REPORT ROBERTS ISLAND AND UNION ISLAND RIPARIAN WATER RIGHTS INVESTIGATION, SAN JOAQUIN COUNTY, CA, JUNE 2008" (PDF).
- W. Francis (1906). Gazetteer of South India. p. 199.
- Shivanasamudra
- Historical Dictionary of Pyongyang, by Justin Corfield, p.227
- "2016 Census Profile - Montréal, Territoire équivalent (Census division)". Statistics Canada. 8 February 2017. Retrieved 2017-03-23.
- "U.S. Census Bureau QuickFacts: New York County (Manhattan Borough), New York; United States". www.census.gov. Retrieved 2019-09-12.
- "2016 Census Profile - Laval, Territoire équivalent (Census division)". Statistics Canada. 8 February 2017. Retrieved 2019-09-11.
- "2016 Census Profile - Richmond (Census division)". Statistics Canada. 8 February 2017. Retrieved 2019-09-11.