These islands result from changes in the course of a river such as interactions with a tributary or the opposing fluvial actions of deposition and/or erosion such as forming a natural cut and meander. Nascent vegetation-free shoals and mudflats may dissipate and shift or build up into such islands through deposition and which may be assisted through artificial reinforcement or natural factors such as, reeds, palms, evergreen trees or willows, to large examples covering many square kilometers, examples of which are given below.
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.
Majuli, in the Brahmaputra River in India, being non-coastal and being between two channels of one river, is recognised by the Guinness Book of World Records as world's largest example, but Ilha do Bananal meets the same qualifications and is much larger.
Umananda, 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 in the Amazon basin and Bangladesh. Another island of comparable size to Umananda, Hatfield Island in the Guyandotte River in the US 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 of permanent-materials homes exist such as Monkey, Friday, Holm and D'Oyly Carte islands, as canalisation reduces the erosion of the islands and in particular limits the height of flash flooding by maintaining substantial "heads" of water through barrages.