The Castlereagh River is located in the central–western district of New South Wales, Australia. It is part of the Macquarie-Castlereagh catchment within the Murray–Darling basin and is an unregulated river, meaning no dams or storage have been built on it to control flows.http://www.water.nsw.gov.au/water-management/basins-and-catchments/castlereagh-catchment . Accessed 20 April 2018 On a map of NSW, the Castlereagh has a distinctive appearance among the north-western rivers for its fish-hook-like shape: from upstream in the north at its confluence with the Macquarie River it extends southwards to a hook-shape, flattened-out at the base, which curves to the right (east and northwards) through to the tip of the hook in the Warrumbungle Mountains at the river's source.
The Castlereagh rises 20 km west of Coonabarabran in the heart of the Warrumbungle mountains at an elevation of about 850 metres."Water Sharing Plan for the Castlereagh Unregulated and Alluvial Water Sources – Background Document for amended plan 2016, NSW Department of Primary Industries – Water, published 2016,'The Castlereagh River Valley' p. 8 From its commencement upstream in the mountains the river initially flows east through the town of Coonabarabran. It then arcs round south and southwesterly to the village of Ulamambri, then south to Binnaway, and SSW to the small town of Mendooran which lies at the eastern side of the river's hook-shape. From here the Castlereagh flows in a westerly direction at the flattened bottom of the hook and then turns north-westerly towards the town of Gilgandra. From Gilgandra it continues to run northwesterly across the plains through Gulargambone and Coonamble, then continuing across expansive plains, with no urban centres, where elevations are less than 200m, until it joins the Macquarie River at its uppermost reaches beyond the Macquarie Marshes. Its confluence with the Macquarie is about 65 km SW of Walgett. Not far from the Castlereagh/Macquarie confluence the Macquarie itself joins the Barwon River at a place about halfway between Walgett and Brewarrina.
Downstream from Gilgandra, the Castlereagh runs through alluvial plains. These are flat landforms with alluvial soil that has been created from sediments being deposited over a long period of time by one or more rivers flowing down from highland regions.World Atlas, 'What is an Alluvial Plain?' https://www.worldatlas.com/articles/what-is-an-alluvial-plain.html Accessed 21 April 2018 Groundwater is found in the alluvial sediments on the plains in the lower Castlereagh catchment and is generally associated with the ancient channel of the river. The Castlereagh has a very sandy bed throughout its course, with especially wide outer banks on the plains. Charles Sturt, after travelling along the Castlereagh in 1828 from Coonamble to its junction with the Macquarie, noted that there was not one pebble or stone to be picked up in its bed, which was dry for the whole length he travelled.see Sturt, Capt Charles, 'Two Expeditions into the Interior of Southern Australia', Volume 1, pub. Smith, Elder and Co London, at chapter 4, section 'Starving condition of the Natives'. https://ebooks.adelaide.edu.au/s/sturt/charles/s93t/
Further upstream, the Castlereagh, Macquarie and Bogan rivers run "more or less parallel as they cross the plains, where creeks and streams break away from the main rivers, making connections between the Macquarie and the Bogan, the Macquarie and the Barwon, and the Castlereagh and the Barwon. As the waterways approach the Barwon River the interconnected streams, as well as lagoons and channels, support extensive flood-dependent woodlands and grasslands". In this extreme northern section of the catchment the floodplain between the Barwon and Castlereagh Rivers is intersected by Womat and Wanourie Creeks, which carry flows from the Barwon to the Castlereagh River during major floods.
Estimates of the length of the course of the Castlereagh River from its commencement to its mouth vary from , 549 km, and 566 km. Over its course it drops over 600 m.
River flows The Castlereagh River has a highly variable flow, and often has from very low flow to no flow at all. The sandy riverbed of the Castlereagh is frequently dry in many places. John Oxley's expedition of 1818 came across the Castlereagh near Coonamble running so high and fast that it was six days until water levels dropped sufficiently to allow the expeditionary party to cross. Ten years later Charles Sturt arrived in the same general area during a ravaging drought to find the Castlereagh dry, and continuing completely dry for miles on end downstream to its junction with the Macquarie River with only occasional waterholes to be found nearby.
Two claims are often anecdotally made about flows in the Castlereagh:
(i) Claim of the fastest flowing inland river in Australia.See for example 'The Sun', 31 July 1936. "Coonamble’s Way" p. 21
(ii) Claim as an underground flowing river.See for example the Coonamble Shire flyer at http://www.coonambleshire.nsw.gov.au/VisitingCoonamble/coonamble.html accessed 24 April 2018 In more recent times, it has also been called an "upside down river".see for example Coonamble Shire, 'Coonamble Visitors Guide', "From the Mountains to the Marshes" p. 2, and www.coonamblerodeoandcampdraft.com.au 2014 guide accessed 24 April 2018
Flow velocity The gradients of the upper reaches of the Castlereagh River and its tributaries are steep, located in the Warrumbungle Mountains and its foothills. In flood events, velocities can be dangerously high – a speed of 3 metres per second has been recorded at Coonabarabran.Warrumbungle Shire, Local Flood Plan, "Characteristics of Flooding", May 2007 p A-4 Additionally, the catchment area of the upper Castlereagh is criss-crossed with many streams and creeks flowing off the foothills, whose floodwater rushes with high velocity into the main river. Flow times downstream can vary greatly depending on the severity of flood. Flood peaks travelling between Coonabarabran and Gilgandra have been recorded taking as little as nine hours to travel the distance, and up to 47 hours. Usually in peak floods, the greater the water volume, the faster the travel times of the peak. However, the flattening of the terrain upstream of Coonamble results in decreased flow velocity (but means a greater area for the flow is needed to avoid flooding).Jacobs Group Pty Ltd, for Coonamble Shire Council,'Data Collection, Review and Flood Study for West Coonamble, Gulargambone and Quambone – West Coonamble Flood Study Report, p. 7. file:///Users/Jo/Downloads/Draft+West+Coonamble+Flood+Study+-+website+version%20(1).pdf Accessed 24 April 2018
Underground flow It is not correct that the Castlereagh River is an underground river (subterranean river) or an 'upside down' river.Nikki Harrington and Peter Cook 'Groundwater in Australia', published by The National Centre for Groundwater Research and Training. http://www.groundwater.com.au/media/W1siZiIsIjIwMTQvMDMvMjUvMDFfNTFfMTNfMTMzX0dyb3VuZHdhdGVyX2luX0F1c3RyYWxpYV9GSU5BTF9mb3Jfd2ViLnBkZiJdXQ/Groundwater%20in%20Australia_FINAL%20for%20web.pdf accessed 21 April 2018, pp. 2–3 However, water that once was in the river bed on the surface does flow through the extensive sands under the riverbed. This water is therefore part of the underground soil, and is not separate from it as a truly underground river is separate from the soil. The fact of water existing under the riverbed was known by Europeans at least in the early part of the last quarter of the 1800s, if not before, as evidenced in statements given to the Water Conservation Commission Inquiry at Dubbo in 1885.SMH 30 May 1885 p 11. See statements of Mr RG Dulhunty and Mr G. Cass
The below-ground sedimentary deposits in the area of the Great Artesian Basin are particularly good water stores (aquifers), and the highly permeable sediments below NSW's western flowing rivers create excellent aquifers.http://www.bom.gov.au/water/regulations/fundingProgram/document/swimps/nsw/nsw_swimp_supplementary_report_2010.pdf. NSW Office of Water, June 2010, at p. 78. Accessed 21 April 2018 A theory of an extensive sub-artesian store of water was proposed by Mr James Samuels and published in the "Empire" of 27 and 28 August 1868, including a view that dry inland rivers would have water under the bed. Some pastoralists in western NSW paid attention and began to sink wells, often near rivers.'Empire' 14 January 1870 This may have contributed to the idea of an underground flowing river in the largely empty above-ground Castlereagh.
The river contains wide alluvial sandbeds. Because the pore space between sand particles is large, water can easily move through the soil layer (i.e. the sand layer). This means the river's sandbeds can be described as highly permeable. (Permeability is measured by calculating the discharge rate at which water will be transported through the pore spaces of the material; the discharge rate being controlled by the hydraulic conductivity of the saturated material).
The Castlereagh River and the ground water of the Great Artesian Basin are highly connected. The water is not flowing as a separate and distinguishable underground river flow, but is moving initially through the permeable alluvial sandbeds and then through paleochannels to deeper groundwater aquifers.
The Castlereagh's alluvial sandbeds make up what is called the "hyperheic zone". This is the region of saturated sediment below, or adjacent to, a river where surface water and groundwater are actively mixing and exchanged. The movement of the groundwater in the Castlereagh through its alluvial sands is said to be a hyporheic process. In the Castlereagh, the further transit of surface water to the aquifers of the Great Artesian Basin is then enabled through the ancient paleochannels underlying the Castlereagh, and this water movement is a large-scale hyporheic process. (Hutchinson and Webster 1998).
The Great Artesian Basin The Castlereagh sits over the Great Artesian Basin (GAB) and within a re-charge area for the waters of the GAB (where the GAB waters are replenished from above ground). Areas of re-charge are divided into zones, called 'sources' and the Castlereagh River is within the Southern Recharge Groundwater Source. This source, together with the Eastern Recharge Groundwater Source, is characterised by better quality groundwater than other zones.
Tributaries The Castlereagh River is joined by dozens of minor tributaries in its journey towards the very upper Macquarie. The Major Rivers Database of NSW lists seventy streams, (including effluent streams that receive water from the river) of the Castlereagh.http://www.environment.nsw.gov.au/vegetation/MajorRivers.htm#cwcma extracted 20 April 2018
On the upper reaches the most important tributaries, originating in the mountains or slopes, and joining the river before Ulamambri are: Shawn, Gundi, Billy Kings, Baby and Terrawinda Creeks. Above Binnaway are Jack Halls, Belar, Spring, Greenbah and Ulinda creeks, and between Binnaway and Mendooran are Chinamans, Greyholme, Piambra and Dinnykymine Creeks.
The section between Mendooran and Gilgandra has many creeks and waterways, and some of the tributaries are, from upper to lower section: Sand Creek and Borambil Creek, Wallumburawang Creek, Rocky Creek, Coondoomea Creek, Gidgenboyne Creek, Boyben Creek, Ryans Creek, Apple Tree Creek, Denmire Creek, Gum Creek, Sallabalah Creek, Rocky Creek; then Quandong Creek, Back Creek and Deep Creek near the area of Breelong, and Jumpers Creek just south of Gilgandra.
Between Gilgandra and Coonamble the tributaries are Ulamogo Creek joining the Castlereagh partway between Gilgandra and Curban, Terrabile Creek just near Curban, Gulargambone Creek at Gulargambone, Wilber Creek between Gulargambone and Combara, and Baronne CreekGoogle Maps