Montana has a great stream access law that allows recreationists to access most streams in the state. Where streams or rivers flow through private land you are allowed to be fishing in the stream as long as you enter through a legal access point and stay within the ordinary high-water mark. Private spring creeks are an exception to the rule where you must pay to fish. Public access sites such as Forest Service, BLM or State Lands make for the easiest places to legally access streams. Also there are restrictions on where motorized craft can be used, so check with FWP to see where they can be used. Private spring creeks are an exception to the rule, and these are pay to fish streams. Abuse of this law could see changes to it, so it is good to be courteous to the land owners. Please do not litter, and keep dogs from harassing livestock or entering private land.
Montana passed a new bill to clear up confusion about accessing from bridges. “The public has access to surface waters by public bridge or county road right-of-way. The Department of Fish, Wildlife and Parks, in cooperation with the affected landowner and county, are responsible for providing public passage around or through a fence preventing such access. A typical access feature would be a stile, gate, roller, walkover, or wooden rail fence.”
"Ordinary high-water mark" means the line that water impresses on land by covering it for sufficient periods to cause physical characteristics that distinguish the area below the line from the area above it. Characteristics of the area below the line include, when appropriate, but are not limited to deprivation of the soil of substantially all terrestrial vegetation and destruction of its agricultural vegetative value. A flood plain adjacent to surface waters is not considered to lie within the surface waters' high-water marks.