The River Irwell is a tributary of the River Mersey in north-west England. It rises at Irwell Springs on Deerplay Moor, approximately 1+1⁄2 miles north of Bacup and flows southwards for 39 mi (63 km) to meet the Mersey near Irlam. The Irwell marks the boundary between Manchester and Salford, and its lower reaches have been canalised and now form part of the Manchester Ship Canal.

The River Mersey is a major river in North West England. Its name derives from Old English and means "boundary river", possibly referring to its having been a border between the ancient kingdoms of Mercia and Northumbria. For centuries it has formed part of the boundary between the historic counties of Lancashire and Cheshire.