There are no additional regulations for fishing within the park boundaries, however all State regulations apply. Since this park is on the boarder of two states please make sure you know where you are fishing at and which regulations apply. Montana Regulations. Boat Fishing. Bighorn Lake is over 70 miles in length and provides thousands of acres of wonderful boat fishing. The lake is between 70 and 450 feet deep in the Montana end. The Wyoming end is 70 feet deep at the state line, 30 feet deep in Horseshoe Bend, gradually becoming shallower going south. South of the causeway at Highway 14A, the river channel expands onto the flats at full lake.