 Starting above Carthage the river briefly divides Lewis and Jefferson Counties before crossing entirely into Jefferson County, where it turns sharply west toward Lake Ontario, flowing past Great Bend, Black River and Watertown.Below Watertown it enters a canyon, well known for its challenging rapids.The river ends at Lake Ontario in the village of Dexter, about west of Watertown, where it empties into the Black River Bay and Marsh, which are parts of the Golden Crescent.