The River Arun is a river in the English county of West Sussex. At 37 miles (60 km) long, it is the longest river entirely in Sussex and one of the longest starting in Sussex after the River Medway, River Wey and River Mole. 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. Turning to the south, it is joined by its main tributary, the western River Rother, and continues through a gap in the South Downs to Arundel to join the English Channel at Littlehampton. 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 Arun gives its name to the Arun local government district of West Sussex.