Mont. Code Ann. § 70-16-201
Owner of land bounded by water
70-16-201 . Owner of land bounded by water. Except where the grant under which the land is held indicates a different intent, the owner of the land, when it borders upon a navigable lake or stream, takes to the edge of the lake or stream at low-water mark; when it borders upon any other water, the owner takes to the middle of the lake or stream.