This section diverts waters from the River Colne at Longford, London, flowing eastwards to the River Crane. For a large part of its early course, it flows alongside its younger "twin", the Longford River. Both waterways have recently been diverted as part of the works associated with the construction of Terminal 5. They skirt the western and southern perimeter of Heathrow Airport then separate at The Two Bridges in Bedfont, to the south east of Terminal 4.
The Duke's river then flows east to join the Crane in Donkey Wood, by Baber Bridge, to the west of Hounslow Heath.
This section is the older of the two, dating from the time of Syon Abbey and predating the purchase of the river with the Syon estate from King James 1 by the Duke of Northumberland around 1605.
The Duke of Northumberland's River can thus be described as a distributary of the Colne and a tributary of the Crane; it is also a distributary of the Crane and a tributary of the Thames.