|Mostly perennial river|
|Name origin: in honour of Lord Castlereagh|
|State||New South Wales|
|Regions||IBRA: Brigalow Belt South, Darling Riverine Plains|
|Districts||Central West, Orana|
|Municipalities||Warrumbungle, Coonamble, Walgett|
|- location||west of Coonabarabran|
|- elevation||630 m (2,067 ft)|
|Mouth||confluence with the Macquarie River|
|- location||west of Walgett|
|- elevation||121 m (397 ft)|
|Length||541 km (336 mi)|
The river rises in the heart of the Warrumbungle mountains and initially flows east through the town of Coonabarabran. It then follows a looping course to the south, west, and finally northwest, joined by 26 minor tributaries, towards its confluence with the Macquarie River; dropping 509 metres (1,670 ft) over its course of 541 kilometres (336 mi). The Castlereagh River has a highly variable flow, and often, low to no flow.
- "Castlereagh River". Geographical Names Register (GNR) of NSW. Geographical Names Board of New South Wales. Retrieved 18 January 2013.
- "Map of Castlereagh River". Bonzle.com. Retrieved 18 January 2013.
- "Macquarie–Castlereagh". Explore the Basin. Murray-Darling Basin Authority. 2008. Retrieved 18 January 2013.
- "Search for the Inland Sea". Exploration. State Library of New South Wales. 2005. Retrieved 13 July 2006.
- "New South Wales in 2010: La Niña brings wettest year in half a century". Annual Climate Summary for New South Wales. Bureau of Meteorology. 4 January 2011. Retrieved 26 September 2011.
- "Castlereagh River catchment" (MAP). Office of Environment and Heritage. Government of New South Wales.
- Castlereagh River Profile sourced from the Murray-Darling Basin Authority.