This article is about river islands as geomorphic formations. For the UK retailer, see
River Island.
"Towheads" redirects here. For the 2013 film, see
Towheads (film).
For islands located completely within a river, see
Ait.
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.
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
Look up towhead in Wiktionary, the free dictionary.
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,[who?] as they consider the
Araguaia River to form two distributaries, and Bananal Island to be the landmass between these two distributed rivers.[citation needed] 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.[citation needed] 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.[citation needed] One-home islands improved by river canals include
Monkey,
Friday,
Holm and
D'Oyly Carte islands.
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.