The River Mersey is a river in the North West of England. Its name is derived from the Anglo-Saxon language and translates as "boundary river". The river may have been the border between the ancient kingdoms of Mercia and Northumbria and for centuries it has formed part of the boundary between the historic counties of Lancashire and Cheshire.
The start of the Mersey is at the confluence of the River Tame and River Goyt in Stockport. It flows westwards through the suburban areas of south Manchester, then into the Manchester Ship Canal at Irlam, becoming a part of the canal and maintaining the canal's water levels. After 4 miles the river exits the canal, flowing towards Warrington where the river widens. It then narrows as it passes between the towns of Runcorn and Widnes. From Runcorn the river widens into a large estuary, which is 3 miles across at its widest point near Ellesmere Port. The course of the river then turns north as the estuary narrows between Liverpool and Birkenhead on the Wirral Peninsula to the west, and empties into Liverpool Bay. In total the river flows 70.33 miles.
A railway tunnel between Birkenhead and Liverpool as part of the Mersey Railway opened in 1886.