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.

Look up towhead in Wiktionary, the free dictionary.

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").

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.