The Penobscot River is the largest river in Maine. The West Branch rises near the Canadian border and flows east and southeast to its junction with the East Branch, or Seboois River, at Medway. The river then flows in a generally southwest course until it empties into Penobscot Bay. Its principal tributaries are the Mattawankeag and the Piscataquis. The Penobscot is navigable for seagoing vessels to tide water at Bangor, 25 miles from its mouth. The river is about 300 miles long.