The Middle Branch Oswegatchie River unites with the West Branch Oswegatchie River near the village of Harrisville, in Lewis County. The river continues downstream as the West Branch Oswegatchie River, until it unites with the Oswegatchie River at a point a short distance west of the village of Talcville. From that point, it is known as "the Oswegatchie," as it flows northerly to its terminus with the St. Lawrence River near Ogdensburg, New York. The last major tributary to the system is flow from the Indian River system after Black Lake. The lower part of the main stem of the river, between Gouverneur and Ogdensburg, is slow-moving, with some rapids that can be handled by experienced canoeists. Important fish species include smallmouth bass, northern pike, musky, channel catfish, walleyes, red-finned mullet, and common carp.
