River Irwell - Natural History

Natural History

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 per cent, 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.

Two Sites of Special Scientific Interest (SSSIs) are located close to the banks of the Irwell, near to its confluence with the River Croal at Moses Gate Country Park near Bolton. The first is at Nob End, a 21.7-acre (8.8 ha) site which has been designated because of its biological interest, based on the predominance of flora typical of limestone grassland including some nationally rare herbs and orchids. Nob End is also designated as a Local Nature Reserve (LNR). The second site is Ashclough, which is a site of geological interest. These two SSSIs are among the 21 found in Greater Manchester. In Salford the river flows through Clifton Country Park and Kersal Dale Country Park, both of which have been designated as LNRs.

Herons, cormorants, mute swans, kingfishers and many species of geese and ducks are regularly sighted on the river. The Manchester Ship Canal near Salford Quays is one of the top ten sites in Britain for diving ducks, providing a winter home to approximately 3,000 common pochard and 2,000 tufted ducks.