The Colne is a river in England which is a tributary of the River Thames. Just over half its course is in south Hertfordshire. Downstream, the Colne is the boundary between Buckinghamshire (specifically the South Bucks district) and London (specifically the London Borough of Hillingdon) and finally between corners of Berkshire and Surrey. On leaving Hertfordshire, the watercourse splits off into several separate branches, a few of which rejoin it, and its main branch flows into the River Thames on the reach above Penton Hook Lock at Staines-upon-Thames.