The River Rother flows from Empshott in Hampshire, England, to Stopham in West Sussex, where it joins the River Arun. At 52 kilometres (32 mi) long, most of the river lies within West Sussex except for the first 10 kilometres (6.2 mi) which lie in Hampshire. The upper river, from its source to Midhurst, has been used to power watermills, with the earliest recorded use being in 1086, when the Domesday survey was conducted. Although none are still operational, many of the buildings which housed the mills still exist, and in some cases, still retain their milling machinery. This upper section is also noted for a number of early bridges, which have survived since their construction in the fifteenth, sixteenth and seventeenth centuries.

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.