River, northwestern England.
It flows west through the southern suburbs of Manchester and is joined by the Irwell in its canalized form as the Manchester Ship Canal. The Mersey is itself modified by the canal as far as Warrington, where it becomes tidal. At Runcorn it is joined by the Weaver, and it forms the Mersey Estuary, the harbour of Liverpool . It enters the Irish Sea after a course of 70 mi (110 km).