I am not that familiar with Montana Law, but I have some knowledge of Minnesota Law, when it comes to Public Waters, and access by the public to these public waters.

Any water structure is measured from the "High Water Mark" of the stream, river, pond or lake. The property owner of any land that is connected to any of the same waters, ends at the high water mark. Any dock or extension into said water structure is not allowed unless approved by the U.S. Coast Guard. All fresh water creeks, streams, rivers, pond, and lakes are under the legal domain of the United States Government. All territories on becoming States, lose jurisdiction of any public waters, inside their border that can be used for transportation of people, materials or supplies (Federal Interstate Commerce Laws). So if it can float a canoe, it is public waters. But you are not allowed to cross private land to get to these public waters. You must cross over public land, or enter where public roads cross these public waters, within the public roads right of way that extends off the both sides of the road for a given distance.

Public land owners are not allowed to change the aquatic subsurface of these public waters, without first receiving permission from the U. S. Coast Guard, and governmental State, County, and Regional Water and Land Conservation Committees and Environmental Impact Committees.

Do not look to the laws and statues from Montana, look else where, in the various agencies of the Federal Government that supersede State Law... on issues of Public Land and Water.