The Adur begins as two branches, west and east, which meet west of Henfield at Betley Bridge.
The western Adur rises at Slinfold and flows through Shipley, where it meets Lancing Brook and on to West Grinstead and Knepp Castle. It is tidal as far north as Bines Bridge south of West Grinstead.
The eastern Adur rises at Ditchling Common, crosses into West Sussex and meets another stream at Twineham. It is fed by the Cowfold Stream at Shermanbury.