The River Mersey is a major waterway in north-west England; the city of Liverpool lies on the north bank of its estuary where the river flows into the Irish Sea. Traditionally the river has formed a political boundary, and this is reflected in its name, which derives from the Old English Maeres-ea, 'boundary river'. In Anglo-Saxon England the Mersey was the boundary between the kingdoms of Northumbria and Mercia; later, it formed the boundary between the counties of Lancashire and Cheshire. Only in 1974 did it cease to be an important boundary marker, with the creation of new counties spanning the river, and the alteration of other boundaries.
The Mersey is formed by the confluence of the Rivers Goyt and Tame at Stockport. It then flows westwards past Cheadle and Sale and through Warrington, and between Widnes and Runcorn, the lowest bridging point on the river. At that point it becomes estuarine in character, several miles broad, flowing past (on the south bank) the canal port of Ellesmere Port, Bebington, Birkenhead and Wallasey, and (on the north bank) Liverpool and Bootle. Here there are major docks.
Two road tunnels and a rail tunnel link Liverpool and Birkenhead/Wallasey.
The River Mersey gave its name to the county of Merseyside, created in 1974 to unite the Liverpool conurbation under one authority. The name also influenced the naming of the Mersey Poets and the Mersey Beat, the 1960s musical style encompassing the early Beatles, Gerry & the Pacemakers, etc.
With decreasing levels of pollution much wildlife has returned to the Mersey. A survey in 2003 discovered a small population of caimen living in the lower reaches of the river.