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.
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 or shoal within a 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".
Largest and smallest
, located in the Brahmaputra River in Assam, India, is recognised by Guinness World Records as the world's largest inhabited riverine island, at.The Encyclopædia Britannica cites another large non-coastal landmass, Bananal Island, located in Tocantins, central Brazil, to be the world's largest river island instead, at.
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, 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.
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.