The Brun rises at the confluence of Hurstwood Brook (draining Wether Edge, Hameldon and supplying the Hurstwood Reservoir) and Rock Water at Foxstones Bridge near the village of Hurstwood. From here, the new river runs northwest towards the town of Burnley.
The river passes through the artificial Rowley Lake near Rowley Hall and collects Swinden Water and the River Don, the latter at Netherwood Bridge.
In Burnley, the Brun travels through Queen's Park and Thompson Park before moving through the town centre, where it is occasionally culverted.
The Brun joins the River Calder close to a roundabout on Active Way in Burnley.
The river or the moorland around it is one of the major candidates for the site of the Battle of Brunanburh in 921, the bloodiest battle of its era and a decisive victory for King Æthelstan but whose location is now forgotten.
In 1856 Thomas T Wilkinson, a master at Burnley Grammar School and antiquary, published a paper suggesting the moors above Burnley as the site of the battle, noting that the town stands on the River Brun. Local folklore told of a great battle at Saxifield during the Heptarchy, re-enforced by the occasional discovery of apparently human bones and iron arrowheads. The village of Worsthorne also had a tradition that the Danes constructed defences when a battle was fought on the moor that bares the same name, and that five kings were buried under tumuli apparent in the area.
Amongst other things, Wilkinson showed that the Heasandford area of Burnley is named for a ford of the River Brun on an ancient trans-pennine route known locally as the long causeway, but in part as the Danes road; the idea of a trans-Pennine route here is strengthened by the discovery of the Cuerdale Hoard at the western end of the route, a stathe on the River Ribble
- Location map: 53°47’29"N, 2°14’51"W
- Jack Nadin (25 April 2003). "On the trail of the elusive River Brun". Burnley Express. http://www.burnleyexpress.net/news/local-news/on-the-trail-of-the-elusive-river-brun-1-1690131. Retrieved 5 June 2012.
- Chris Gee (14 January 2012). "Burnley polluted river clean up hailed as 'remarkable transformation'". Lancashire Telegraph. http://www.lancashiretelegraph.co.uk/news/burnley/9472017.Burnley_polluted_river_clean_up_hailed_as__remarkable_transformation_/. Retrieved 5 June 2012.
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- Wilkinson, Thomas T (1857), Transactions of the Historic Society of Lancashire and Cheshire, Volume 9, pp. 21–41
- National Monuments Record: No. 45325 – Monument No. 45325