Mont. Code Ann. § 76-5-106

Parts 1 through 4 of this chapter do not extend to any obstruction in the flood plain or floodway of a watercourse or drainway where the drainage area above the watercourse or drainway, either within or outside the state, is less than 25 square miles in extent unless a particular watercourse or drainway is expressly declared to be within the coverage of parts 1 through 4 by order of the department.