The source of the Arun is a series of small brooks (known locally as ghylls or gills) in the St Leonard's Forest area, to the east of Horsham. It flows through Horsham to the west and at Nowhurst is joined by North River (also known as the River Ockle), whose source is the heights of Leith Hill and Holmbury Hill in Surrey.
The Arun flows through Arundel and past its castle. Its main tributary is the River Rother (the westerly of the two rivers of that name in Sussex). The Arun flows into the English Channel at Littlehampton, having run for approximately 25 miles from source to the sea. It is tidal as far inland as Pallingham Quay, 18¼ miles upstream from Littlehampton.
The Arun is one of the faster flowing rivers in Britain.
As early as 150 AD, a river believed to be the Arun was recorded in Ptolemy's Geographia as the Trisantonis. Later records substantiate the connection. Trisantonis is thought to be a British language word for 'the trespasser', indicating the river's tendency to flood land near to the river; the same origin as the River Trent. There is also a theory that the Arun may have been known as the Trisantonis in its lower reaches close to the sea, but known as the Arnus (from the British Arno meaning "run" or "flow").
In the Middle Ages the river was known as the Tarrant, evidenced by Tarrant Street in modern Arundel. How the river became the Arun is unknown; it may have been an alternative name, or a name for part of the river, or it may be named after Arundel (a town generally assumed though to be named from the river).
The mouth of the river has not always been at Littlehampton. Until the later fifteenth century it joined the River Adur at Lancing some ten miles to the east before entering the sea. This estuary became blocked with shingle by the eastward drift of the tides, pushing the Adur towards Shoreham-by-Sea, while the Arun broke out at Worthing, Goring and Ferring at various times, until it formed its present estuary at Littlehampton between 1500 and 1530.
The Arun was linked to the canal network by the Wey and Arun Canal, opened in 1816 and closed in 1871, which is currently being renovated. The river had been made navigable to New Bridge on the A272 between Wisborough Green and Billingshurst by 1787, by the Arun Navigation Company. By 1791 ships of 200 tons could reach Arundel, where goods were transferred to sailing barges which could use wind and tide to travel upriver. There was no towpath on the river. The river was a toll-free navigation as far as Pallingham, by ancient royal charter, causing financial difficulties for the navigation company, which was obliged by Act of Parliament to maintain the river.
The Rother Navigation joined the Arun near Stopham Bridge, a fine mediæval stone bridge built in AD 1423, which carried the heavy traffic on the A283 road until a new bridge was built beside it in the 1980s. The central arch was raised in 1822 to allow masted barges to pass. Littlehampton and its harbour were guarded from naval attack by Littlehampton Redoubt on the western bank at the mouth of the river, completed in 1854, which is now screened from the open sea by Climping sand dunes. This fort replaced a seven gun battery on the east bank built in 1759, which now has a recreational crazy golf course on top of it.
The river was abandoned as a navigation by a warrant issued as part of the Railway and Canal Traffic Act of 1888. The River Lark in Suffolk was the only other river navigation abandoned at that time.
| ("Wikimedia Commons" has material|
about River Arun)
- Submerged Palaeo-Arun River: Reconstruction of Prehistoric Landscapes Report by Imperial College London and English Heritage.
- Victoria County History of Sussex Index of Volume 5 Retrieved 2009-10-25
- P A L Vine, London's Lost Route to the Sea, Middleton Press, 1996, page 20
- P A L Vine, London's Lost Route to Midhurst, The Earl of Egremont's Navigation pp. 51-54
- National Heritage List England no. 1226929: Stopham Bridge, Pulborough (Historic England)
- The Canals of Eastern England, (1977), John Boyes and Ronald Russell, David and Charles, ISBN 978-0-7153-7415-3
- R H Goodsall, The Arun and Western Rother
- P A L Vine, London's Lost Route to the Sea, Middleton Press, 1996
- P A L Vine, London's Lost Route to Midhurst, The Earl of Egremont's Navigation