The River Mersey is a river in north-western England. The river's name comes from Anglo-Saxon Mǽres-ēa = "border river", likely because it was the border between Mercia and Northumbria. It is the traditional border between the historic counties of Cheshire and Lancashire
From Stockport it flows near Didsbury, Stretford, Urmston, Flixton, and then at Irlam it flows into the Manchester Ship Canal, which canalised the River Irwell to this point. The course of the Mersey has been obliterated by the Canal past Hollins Green to Rixton although the old river bed can be seen at Warburton; at Rixton the River Bollin enters the Canal from the south and the Mersey leaves the Canal to the north, meandering through Woolston, where the Ship Canal Company's dredgings have formed a nature reserve (Woolston Eyes), and Warrington. It is tidal from Howley Weir in Warrington, although high spring tides often top the weir.
West of Warrington the river narrows, and passes between the towns of Runcorn and Widnes, in Cheshire, through the Runcorn Gap. The Manchester Ship Canal also flows through the Gap, along the southern bank of the river.
The Runcorn Gap is currently bridged by Runcorn Bridge and Runcorn Railway Bridge, while a project known as Mersey Gateway to build a new road bridge over the Mersey east of the existing bridges is currently under consideration, and has received some government support.
From the Runcorn Gap, the river widens into a large estuary, which is seven miles wide at its widest point near Ellesmere Port. The course of the river then heads north, with Liverpool to the east and the Wirral Peninsula to the west. The Manchester Ship Canal continues along the Cheshire bank of the river as far as Eastham Locks, where it enters the river. The eastern part of this estuary is much affected by silting, and part of it is marked on modern maps as dry land instead of as tidal. These wetlands are of importance to wildlife, and are listed as a Ramsar site.
The estuary then narrows to flow between Liverpool and Birkenhead, where it is constricted to a width of 1.2km (0.75 miles), between Albert Dock and the Woodside ferry terminal. It then flows into Liverpool Bay on the Irish Sea, after a total course of around 70 miles.
The conurbation on both sides of the river in this area is known as Merseyside.
Two road tunnels run under the Mersey at Liverpool: the older Queensway Tunnel (opened 1934) connecting with Birkenhead, and the Kingsway Tunnel (opened 1971) connecting with Wallasey. There is also a railway tunnel dating back to the 1880s, which carries passenger services on the Wirral Line of the Merseyrail franchise.
Water quality in the River Mersey has been severely affected by industrialisation in the region, and in 1985, the Mersey Basin Campaign was established to improve water quality and encourage waterside regeneration. In 2002, oxygen levels that could support fish along the entire length were witnessed for the first time.