When Ptolemy wrote his Geography around 150 AD, the Arun was called the Trisantonis, with later accounts using the same name. Trisantonis is thought to be a Brythonic word for 'the trespasser', indicating the river's tendency to flood land near to the river. Trisanto translates directly as 'one who goes across'. 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 Brythonic 'Arno' meaning run, go, or flow) in its upper reaches. It is possible that the town of Arundel may mean Arno-dell, or dell of the flowing river. By the Middle Ages the river was known as the river of Arundel, the Arundel river, or the high stream of Arundel. An alternative name, the Tarrant (derived from Trisantona), is, however, attested in 725 and 1270, and is reflected in the road name Tarrant Street, one of the main roads running through the town roughly parallel to the river. The first use of the modern name was recorded in 1577, but the alternative names of Arundel river or great river continued to be used for many years.
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 lower portion of the river, from the sea to Ford, was navigable in the eleventh century at the time of the Norman conquest. In the sixteenth century, Henry FitzAlan, 19th Earl of Arundel built wharves at Ford, and improved the river channel below there, so that the town became a port. Over the 30 years from 1544, he also improved the river as far upstream as Pallingham Quay. Although the work involved a number of flash locks, which were not very successful, no tolls were charged for its use, and vessels of around 15 tons were used to carry timber. Attempts to make the river navigable up to Newbridge in the early sixteenth century were not successful.
An Act of Parliament was obtained in 1732, the main emphasis of which was the improvement of "the harbour of Littlehampton, called Arundel Port", but improvements to the first 5.75 miles (9.25 km) of the river, from the sea to Arundel, were also authorised. Commissioners were appointed, with powers to erect piers and to cut a new channel to the sea through a sand bar. The Act allowed them to charge tolls for use of the facilities, and once the initial costs had been repaid, one half of the tolls were to be used to maintain the harbour and the river channel up to Arundel. Although most ships were of 30 or 40 tons, ships of up to 100 tons could reach Arundel as a result of the work, and trade improved.
The next Act to affect the river was obtained by a group of local men in 1785. Under the Act, the proprietors were empowered to make the river navigable for 30-ton barges up to Newbridge. They had no jurisdiction over the river from Arundel to Houghton bridge, and could not charge tolls for use of the river up to Pallingham. There were 31 members of the proprietors, who could raise £10,000 by issuing 100 shares worth £100 each. Day-to-day oversight of the affairs of the navigation were managed by three proprietors, with a half-yearly meeting of the larger group. The purpose of the navigation was to carry coal, chalk and lime upstream, and agricultural produce in the other direction. Rather than improve the river channel, the navigation upstream of Pallingham consisted of a separate channel, containing three locks, and an aqueduct which carried the navigation over the river at Orfold. The journey below Pallingham was made 3 miles (4.8 km) shorter by cutting a new channel between Coldwaltham and Hardham. This involved the construction of three more locks and a 375-yard (343 m) tunnel. The Pallingham to Newbridge section opened on 1 August 1787, while the Hardham cut was completed in mid-1790. The cost of the work was around £16,000.
There were two proposals to extend the navigation at this time. The first was for a canal to North Chapel, to the north of Petworth, in 1791, and the second was for a canal to Horsham in the following year. The route was surveyed by John Rennie, who estimated that it would cost £18,133 to build, but negotiations with the existing proprietors failed, and the scheme was dropped in 1794. Meanwhile, a second Act of Parliament was obtained by the harbour commissioners in 1793, as there was serious silting of the estuary. Groynes were constructed and the existing piers were made longer. In addition, a towpath was built from the mouth of the river up to Arundel. The Act stated that the capital borrowed to finance the harbour under the previous act had been repaid, and that tolls would all be used for maintenance of the harbour and river up to Arundel, once further borrowings had been repaid. Because the inhabitants of Arundel had spent £28,300 on the harbour, boats which belonged to the port of Arundel did not have to pay any tolls. As a result of the works, the port of Arundel enjoyed its most prosperous period for the next thirty years, with ships of 200 and 300 tons able to reach the town on spring tides. Facilities improved, and there were four docks by 1840.
Greatham Bridge, with the navigable span on the extreme right
Payment of dividends to shareholders began in 1792, and over the next five years, tolls raised an average of £893 per year and the dividend was 3.1 per cent. At this time, George Wyndham, 3rd Earl of Egremont was buying shares and having obtained one third of them, he became chairman of the company. He then stopped the payment of dividends so that the borrowed capital could be paid off more quickly. Apart from an interim payment in 1821, dividends were not reinstated until 1830. In the 1790s Wyndham was responsible for the canalisation of the River Rother which joins the Arun at Stopham, and he also promoted the Wey and Arun Canal, which was seen as part of a larger scheme to link London to Portsmouth, an idea which had been contemplated several times since 1641. He chaired a meeting held at Guildford on 1 June 1811, at which it was decided to press ahead with the canal, and put up £20,000 of the initial £90,500 estimated cost. The canal opened in September 1816, but the estimated 100,000 tons of traffic passing between London and the dockyards at Portsmouth, and the 30,000 tons of local traffic, were far too optimistic, with actual traffic averaging around 15,000 tons per year throughout its life.
The London to Portsmouth route was to be completed by the Portsmouth and Arundel Canal, in which Wyndham and the Cutfields, who also held many shares in the Arun Navigation, were both significant subscribers. This was authorised by Act of Parliament in 1817, and an agreement was reached that the Arun would be improved to aid through traffic. Nevertheless, no work commenced on the Arun until the proprietors were sure that the Ford section of the new canal would actually be built. Once they were convinced, they obtained an Act of Parliament in May 1821, and the engineer James Hollingworth oversaw the improvements, which were completed in mid-1823. The work involved improving the depth and width of the channel, and some alterations to bridges and locks to make their size more uniform. The company borrowed £3000 to finance the work, which cost around £5000 in total. The loans had been repaid by 1831, and the work allowed barge sizes to be increased from 30 tons to 40 tons, with the result that business improved.
Traffic increased, as shown by the number of boats belonging to residents of Arundel. There were 13 in 1801, which had increased to 15 by 1803, with a total tonnage of 266. A timber merchant called John Boxold owned barges in 1815 and 1832, while in 1820, a company began running regular freight services to London, using three barges based near the town quay. By 1823 they had ten barges, which had reduced to seven by 1830, and barges ran twice-weekly to Chichester, London, Midhurst, and Petworth. The company was variously called Seward and Co., The Arundel Barge Co., and several other names.
From the 1840s, use of the river declined, as a result of competition from the railways, and changes in coastal shipping. Littlehampton grew in importance as a port and after years of resistance by the people of Arundel, the customs house was moved there in 1864. The Mid-Sussex Railway opened their line from Horsham to Pulborough and Petworth in 1859, which was extended to Ford and Littlehampton in 1863. Receipts from tolls had peaked at £2044 for the five years from 1835 to 1840, when a dividend of 11.8 percent was paid, but dropped quickly, raising just £389 for years between 1870 and 1875, when the dividend was 1 percent.
By 1852, the barge service to London only ran once a week, and it had ceased altogether three years later. Most vessels reaching Arundel were coasters rather than barges by 1886, and just 20 ships used the facilities that year. The Wey and Arun Canal closed in 1871. The proprietors of the upper river ceased to maintain the navigation from 1 January 1888, and the last barge passed through Hardham Tunnel on 29 January 1889. 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. The Board of Trade issued a closing order in 1896, and after that, there was no navigation authority responsible for the upper river. However, traffic did not cease entirely.
Fifteen or twenty barges were still using the river in the 1880s, although the upper reaches were no longer accessible. Arundel docks silted up between 1875 and 1896. In 1898, the London, Brighton and South Coast Railway, who by this time were the owners of the railway from Horsham to Littlehampton, drilled down into the tunnel where the main line and the branch to Midhurst crossed its course, and poured tons of chalk into the tunnel to stabilise it. A trade in chalk and lime extracted from Amberley chalk pits continued into the early twentieth century. Some ships were towed to Arundel by paddle tugs, and imports of salt, timber and coal for the gasworks continued. Arundel was visited by its last steamer in 1914, and the last sailing vessel to reach the port did so three years later. Passage of larger craft upstream was hindered by the construction of a swing bridge at Littlehampton in 1908, and prevented by a fixed railway bridge at Ford built in 1938. As freight traffic disappeared from the river, Edward Slaughter, who later became part of the company of Buller and Slaughter, was hiring pleasure craft by 1903, and the company was still doing so in the 1990s.
Authority for the river remains much as it was after 1896, with the Littlehampton Harbour Board responsible for the section from the mouth up to Arundel Bridge, and no navigation authority for the river above that, although the Environment Agency have responsibility for its drainage functions. There are nine bridges with a minimum navigable headroom of between 8 feet (2.4 m) and 5 feet (1.5 m) at high water. The river is tidal to Pallingham Quay, 25.5 miles (41.0 km) upstream from the sea at Littlehampton, and flows at 4 to 6 knots (7.4 to 11.1 km/h), making it one of the fastest flowing rivers in the country. The tidal range at Littlehampton is 17 feet (5.2 m) at spring tides and 8.8 feet (2.7 m) at neap tides. High tide occurs 15 minutes later than high water at Dover, and high water at Pulborough is four hours later than at Littlehampton.
The Arun & Rother Rivers Trust (ARRT) is a charity set up in 2011 with objectives around education, fisheries, biodiversity, access and pollution amongst other issues.
The Wey and Arun Canal is being restored by the Wey and Arun Canal Trust, which was set up in the 1970s. The Wey and Arun Canal technically ended at Newbridge, but the restoration will include the Arun Navigation section down to Pallingham to reach the River Arun. For many years, the Solent and Arun Branch of the Inland Waterways Association organised an annual cruise on the river to ensure that the navigation rights were maintained. Responsibility for its organisation has now been passed to the Wey and Arun Canal Trust.