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, under Watercourses.
Mont. Code Ann. § 70-18-205
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.