The River Irwell is a 39-mile (63 km) long river which flows through the Irwell Valley in North West England. Its source is at Irwell Springs on Deerplay Moor, approximately 1.5 miles (2.4 km) north of Bacup. It forms the boundary between Manchester and Salford.
The Irwell was joined by the River Mersey at Irlam, whence the combined watercourse was known as the River Mersey. When the Manchester Ship Canal was constructed in the 1890s, it joined the Irwell at Hulme in Manchester (at Ordsall in Salford). Consequently the River Irwell loses its identity at Hulme/Ordsall.
Like its smaller sisters, the River Irk, River Medlock, Shooter's Brook and the River Tib, it was heavily used and abused as a source of water supply and power and as a means of disposal of foul effluent.
A report in 1848 provided the information that the Irwell, Medlock and Irk were together fed by rain falling on a land area of sbout 163,000 acres. About 134,700 acres drained into the Irwell, 11,300 into the Medlock, and 17,000 into the Irk.