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Foyle (foil), river, c.10 mi (16 km) long, formed by the junction of the Mourne and Finn rivers at Strabane, which is on the border between Limavady and Derry dists., W Northern Ireland. It flows northeast through the city of Derry to Lough Foyle, a navigable inlet of the Atlantic Ocean c.15 mi (25 km) long. With its tributaries, the Foyle drains c.1,100 sq mi (2,850 sq km). There are valuable freshwater fisheries on the Foyle.