A fluvial island is an island located within a river or estuary.

Marajó is an island located at the mouth of the Amazon River in Brazil. It is part of the state of Pará. Marajó is the largest fluvial island in the world, and the second largest island in South America (the largest island in S, America is Isla Grande de Tierra del Fuego lying to the south of the mainland of Chile and Argentina). Marajó is huge: with a land area of 40,100 square kilometres (15,500 sq mi) it is comparable in size to Switzerland. It is approximately 295 kilometres (183 mi) long and 200 kilometres (120 mi) wide.

More Info: en.wikipedia.org