In the 17th and 18th centuries, the Irwell's lower reaches were a trading route that became part of the Mersey and Irwell Navigation. In the 19th century, the river's course downstream of Manchester was permanently altered by the construction of the Manchester Ship Canal which opened in 1896. The canal turned Manchester and Salford into a major inland seaport and led to the development of Trafford Park which became the largest industrial estate in Europe. Further changes were made in the 20th and 21st centuries to prevent flooding in Manchester and Salford, such as the Anaconda Cut in 1970 and the River Irwell Flood Defence Scheme in 2014.
The river became severely polluted by industrial waste in the Industrial Revolution, but in the second half of the 20th century a number of initiatives were implemented to improve water quality, restock it with fish and create a diverse environment for wildlife. Stretches of the river flowing through Manchester and Salford have attracted large-scale investment in business and residential developments, such as Salford Quays, and other parts have become important wildlife havens. The Irwell is used for recreational activities, such as pleasure cruising, rowing, racing and fishing.
From its source to the confluence with the River Mersey the Irwell is about 39 miles (63 km) long. Rising on the moors above Cliviger, it flows south through Bacup, Rawtenstall, Ramsbottom and Bury before merging with the River Roch near Radcliffe. Turning west, it joins the River Croal near Farnworth before turning southeast through Kearsley, Clifton and the Agecroft area of Pendlebury. It then meanders around Lower Kersal and Lower Broughton. It bisects Salford and Manchester, joining the rivers Irk and Medlock, and then turns west toward Irlam, as part of the Manchester Ship Canal. Its course ends just east of Irlam, where it empties into the Mersey.
Until the early 19th century the Irwell was well stocked with fish and other wildlife, with people living near Manchester Cathedral using its water for drinking and other domestic purposes. However, during the Industrial Revolution, increasing levels of pollution caused by waste products discharged into the river by local industries proved fatal to wildlife, with fish stocks disappearing completely by about 1850. This situation abated somewhat during the 20th century, with a slow improvement in water quality leading to fresh populations of roach, bream and chub, and sightings of brown trout have become increasingly common.
Problems with water quality in some of the former Manchester Docks basins became apparent with the redevelopment of Salford Quays. Years of runoff from sewers and roads had accumulated in the slow running waters of this area and decomposition of organic matter was causing oxygen depletion. In 2001, a compressed air injection system was introduced. This raised the oxygen levels in the water by up to 300%, improving the water quality to such an extent that the number of invertebrate species present, like freshwater shrimp, increased by more than 30. Spawning and growth rates of fish species such as roach and perch have also increased, and are now amongst the highest in England.