It rises at Irwell Springs on Deerplay Moor, approximately 1+1⁄2 miles (2.5 kilometres) north of Bacup and flows southwards for 39 mi (63 km) to meet the Mersey near Irlam Locks.

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.

[3] 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.

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.

The first is at Nob End, an 88 800 m2 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.

[12] The glaciers of the Pleistocene epoch radically re-shaped the landscape and then retreated, leaving behind deposits of sand, pebbles and boulder clay that formed the fluvioglacial ridges of the lower Irwell Valley.

This comprises Lower Coal Measures overlying Millstone Grit, both of which are classified as minor aquifers which will only hold relatively small amounts of water.

[22] There have been isolated finds of artefacts characteristic of this period along the Irwell valley, and a possible hunting site was excavated at Prestwich Golf Course in 1982, which produced a quantity of Late Neolithic and Early Bronze Age flints.

[26] For four hundred years the Pax Romana brought peace, but their withdrawal in AD 410 left the tribes at the mercy of the Saxons, who renamed the town Manigceastre.

"[3] In 1862 the Scottish geologist Hugh Miller wrote about the Irwell, in his book First Impressions: The English People, describing it as: The hapless river—a pretty enough stream a few miles higher up, with trees overhanging its banks, and fringes of green sedge set thick along its edges—loses caste as it gets among the mills and the printworks.

There are myriads of dirty things given it to wash, and whole waggon-loads of poisons from dye-houses and bleachyards thrown into it to carry away; steam-boilers discharge into it their seething contents, and drains and sewers their fetid impurities; till at length it rolls on—here between tall dingy walls, there under precipices of red sandstone—considerably less a river than a flood of liquid manure, in which all life dies, whether animal or vegetable, and which resembles nothing in nature, except, perhaps, the stream thrown out in eruption by some mud-volcano.

In 1878 he became the only civilian ever to be awarded the Albert Medal (first class),[37] His final rescue was on Whit Monday in 1889, when he saved a young boy from a particularly sewage-laden section of the river.

[47]In 1980, a scheme for improving a section of the river between Littleton Road and Adelphi Weir in Salford for boating purposes was included in Greater Manchester Policy Committee's capital programme for 1981/82.

[48] Later that year it was reported that hundreds of trees and shrubs were to be planted along the banks of the Irwell between Broughton and Pomona Dock in a £650,000 "green finger" scheme to "bring the countryside into the heart of industrial Salford".

[49] The dossier outlining the scheme, prepared by Salford's Technical Services Officer, emphasised the "bleak character" of the Irwell Valley with "many constructions by the river that are decaying and rotting" and said "the main problems are caused by an excess of ammonia and a high organic content from sewage effluents which discharge into the water upstream."

The Manchester Ship Canal Company also announced a £70 million redevelopment scheme for Pomona Docks in Salford, to include a marina, homes, offices and other commercial developments.

[60] This was not universally welcomed as, by September of that year, a retired chemical engineer claimed that the pleasure boat was "leaving clouds of methane in its wake as it disturbed sediment on the river bed".

[61] However, in 1991 a feature article appeared in the magazine Lancashire Life extolling the virtues of the cruise, and stating that the Irwell "once thought a liability is in fact a major asset to urban regeneration ... now the twin cities compete to befriend her, to dress her in finery and proudly introduce her back into society.

The box-girder and steel cable construction was designed by Spanish architect Santiago Calatrava to represent a ship in full sail[63] as the centrepiece of the £50M Chapel Wharf Regeneration Scheme, which was expected to breathe new life into a run-down area of Salford and to create up to 8,000 new jobs.

[66] In July 2000 there was a setback for the clean-up programme when a pollution incident affected approximately 2+1⁄2 miles (4 kilometres) of the river between Rawtenstall and Ramsbottom, causing the death of more than 36,000 fish.

[71] In 1721, Parliament authorised the alteration with the Mersey and Irwell Navigation Act, and by 1736 work had been completed by creating eight canal locks along the 20-mile (32-kilometre) route from Warrington to Manchester, allowing access to boats of up to 51 tonnes.

[76] Over the years both trade and rivalry continued to grow with each company offering reduced freight charges or special rates and concessions to gain business from the other.

In December 1816 rapidly thawing snow caused the river to flood, sweeping away a considerable amount of property including building materials and livestock.

[87] To alleviate such problems two flood storage basins with a capacity of 650,000 cubic metres (850,000 cu yd) have been constructed to the west of Littleton Road, Kersal, at a cost of around £11 million.

[88] The defences proved successful on 22 January 2008, when the worst flooding to hit the region in thirty years caused the Irwell to burst its banks further downstream in the centre of Manchester.

[89][90] On 2 February 2018 the Environment Agency announced the completion of A£10 million extension to the flood defence scheme which will protect almost 2,000 homes and businesses and has created more than 5 hectares of high quality urban wetland habitat.

[91] A large urban regeneration project, The Salford Quays Development Plan, was published in 1985 and work began a few years later to redevelop the site for residential, business and leisure purposes.

[92] There are further plans to "turn the tide on the years of neglect and once again embrace the River Irwell as a fundamental part of Manchester and Salford's heritage and future economic and social growth.

The Guidance sets out the guidelines that the councils will use as a material consideration in determining applications for planning permission and other matters in the Irwell City Park area.

At the turn of the 20th century, rowing was very popular in the area with many local clubs such as Nemesis, Prince of Wales, Minerva, Didsbury and Agecroft all competing regularly.