If a stream navigable or not navigable, in forming itself a new arm, divides itself and surrounds land belonging to the owner of the shore and thereby forms an island, the island belongs to such owner.
Mont. Code Ann. § 70-18-205
Island formed by division of stream
En
Official source: Montana Code Annotated (Montana Legislature). Reproduced from public-domain Montana statutes; confirm against the official source for the current text. Not legal advice.