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The River Irwell is a river in the counties of Lancashire and Greater Manchester in England. It is one of several rivers that helped facilitate the Industrial Revolution in North West England.

With its source at Cliviger, Lancashire, the River Irwell flows through (and separates) the city centres of Manchester and Salford, before joining the River Mersey.

Rising on the moors at Cliviger it flows south through Bacup, Rawtenstall, Ramsbottom and Bury before merging with the River Roch near Radcliffe. Turning west it is joined by the River Croal near Farnworth and runs south east where it meanders around the centre of Manchester, joining the rivers Irk and Medlock. Again turning west, from Salford until it meets the Mersey south of Irlam, its route was altered in the late 19th century to form part of the course of the Manchester Ship Canal.

In the late 17th century, the Warrington businessman Thomas Patten, had made the River Mersey navigable as far as Warrington and suggested that there would be significant commercial value in extending this along the Irwell as far as Manchester. 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 route from Warrington to Manchester, allowing access to boats of up to 50 tons. The waterway played a central role in the cotton industry of the 18th century that spear-headed the Industrial Revolution.

When James Brindley built the Bridgewater Canal, the task of crossing the Irwell Valley was one of the main obstacles he faced. His solution was to build a canal-carrying bridge across the river, the world's first commercially driven aqueduct. The bridge consisted of three arches, and measured 12 metres high, 200 metres long and 11 metres wide. Upon its opening in 1761 it earned the nickname of "the Castle in the Air" and proved hugely successful in the industry of the area.

As the 19th century progressed, the increasing need for large freight carriers led to Liverpool's dominance as a port, and Manchester became increasingly reliant on its Merseyside neighbour for its export industry. A solution was to build the Manchester Ship Canal. The ship canal opened in 1894, by the expansion of the route of the Irwell and Mersey. Although it came too late to save the cotton industry that had made the region the centre of the Industrial Revolution, it transformed Manchester into England's third largest port, despite being 40 miles inland. As the canal was built, it became clear that Brindley's famous aqueduct would have to be demolished, as it allowed insufficient headroom for the freighters that the canal would carry. Fortunately, in 1896 the councillors of Eccles paid to have the aqueduct moved to the spot it occupies today, alongside the canal.

Instead, the Bridgewater Canal is now carried over the Irwell/Ship Canal by the equally celebrated Barton Swing Aqueduct, which was completed in 1893 with the novel idea of "opening" by rotating 90 degrees to allow ships to pass.

Two Sites of Special Scientific Interest (SSSIs) are located close to the banks of the Irwell when it flows between Prestolee and Little Lever. The first is Ashclough a 5.8 hectare (14.3 acre) site which comprises the steep banks of the river. It has been designated because of its geological interest, primarily because it is the best available site displaying Ashclough Marine Band and its associated strata.^[1]

The second site is Nob End, an 8.8 hectare (21.7 acre) site which has been designated because of its biological interest. The interest is based on the predominance of herbs typical of limestone grassland. This is because limestone grassland is not common in the surrounding area meaning that many of these species are rare.^[2] These two SSSIs are among the 21 found in Greater Manchester.

Moving upstream from the Mersey Confluence, the tributaries include the following: -

• Platts Brook
• Bent Lanes Brook
• Salteye Brook
• River Medlock
• River Irk
• River Croal
  • River Tonge
    • Bradshaw Brook
    • Astley Brook
    • Eagley Brook
• River Roch
  • River Spodden
  • River Beal
• Pig Lees Brook
• Kirklees Brook
• Dearden Brook
• River Ogden
• Whitewell Brook
• Limey Water

Despite its industrial past, fish stocks in the River Irwell are improving and brown trouts are becoming an increasingly common sight. Coarse fish now predominate throughout the river's course and beyond into the Manchester Ship Canal itself.