The longest rivers are the Vistula (),  long; the Oder () which forms part of Poland's western border,  long; its tributary, the Warta,  long; and the Bug, a tributary of the Vistula,  long. The Vistula and the Oder flow into the Baltic Sea, as do numerous smaller rivers in Pomerania. Poland's lengthy waterways have been used since early times for navigation; the Vikings ventured up the Polish rivers in their longships. In the Middle Ages and in early modern times, the shipment of tangible goods down the Vistula toward Gdańsk and onward to other parts of Europe took on great importance.