From the series of small streams that form its source in the area of St Leonard's Forest in the Weald, the Arun flows westwards through Horsham to Nowhurst where it is joined by the North River.

It is one of the faster flowing rivers in England, and is tidal as far inland as Pallingham Quay, 25.5 miles (41.0 km) upstream from the sea at Littlehampton.

The work was carried out by Henry FitzAlan, 19th Earl of Arundel, who made the upper section toll-free.

Harbour commissioners managed the lower river from Arundel to the sea from 1732, and major improvements to keep the estuary free from silt were sanctioned by an act of Parliament[which?]

One, which included three locks and passed through Hardham Tunnel, was built to avoid a large bend near Pulborough.

These two canals were an attempt to provide an inland route between London and Portsmouth, but were not as successful as the proprietors hoped.

Until the later fifteenth century it joined the River Adur at Lancing some ten miles to the east before entering the sea.

[9] 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.

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.

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.

[16] 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.

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 work involved improving the depth and width of the channel, and some alterations to bridges and locks to make their size more uniform.

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.

[21] The river was abandoned as a navigation by a warrant issued as part of the Railway and Canal Traffic Act 1888.

[22] The Board of Trade issued a closing order in 1896, and after that, there was no navigation authority responsible for the upper river.

Some ships were towed to Arundel by paddle tugs, and imports of salt, timber and coal for the gasworks continued.

[24] 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.

[26] 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.

[2] It rises as a series of streams, known locally as ghylls or gills, to the east of Horsham, in St Leonard's Forest.

It flows westwards, along the southern boundary of Horsham and turns briefly to the north to skirt Broadbridge Heath.

[32] Pallingham Quay Farmhouse, another Grade II listed building dating from the 18th century,[33] is on the west bank of the river just before its junction with the Arun Navigation cut.

[31] Continuing southwards, the river passes the gallops which are part of Coombelands Racing Stables, situated on the eastern bank,[31] and Park Mount, a motte and bailey dating from the time of the Norman conquest.

It was damaged by army lorries in the Second World War, but has been repaired,[35] and the heavy traffic on the A283 road was diverted onto a new bridge just upstream of it in the 1980s.

Below the bridge is a small island, after which an artificial cut built to avoid the circuitous route of the River Rother Navigation heads westwards.

The river now discharges over a weir at the site of the former Hardham corn mill to join the Arun a little further downstream, and the junction is followed by another small island.

[42] The present building consists of many different components, dating from the late eleventh century through to the nineteenth, and is Grade I listed.

The reasons for the quality being less than good include sewage discharge affecting most of the river, physical modification of the channel, and run-off of nutrients from agriculture and land management.

Like most rivers in the UK, the chemical status changed from good to fail in 2019, due to the presence of polybrominated diphenyl ethers (PBDE), perfluorooctane sulphonate (PFOS) and mercury compounds, none of which had previously been included in the assessment.