North Sea inlet on the east coast of England, one of the major deepwater estuaries of the United Kingdom. The River Humber originates at the confluence of the Rivers Ouse and Trent and forms the historic boundary between the counties of Yorkshire and Lincolnshire. The Humber is about 40 miles (64 km) long, extends west to east, and, with associated rivers and canals, drains 9,550 square miles (24,750 square km). The estuary itself is more than 0.75 mile (1.2 km) across at its farthest inland point and widens to more than 7 miles (11 km) near its mouth; there Spurn Head, a sand and shingle spit with lighthouse, lifeboat station, and bird sanctuary, extends into the estuary. The great width of the estuary has prevented unified economic development of its traditionally quite different north and south banks. The Humber Bridge (opened in 1981), spanning the estuary, was constructed chiefly to aid further development. Measuring 4,626 feet (1,410 metres) in length, it was the longest suspension bridge in the world when it was built and remains the longest in the United Kingdom. The River Humber is lined by the major ports of Kingston upon Hull, Grimsby, and Immingham.