The Colne is a river

River

A river is a natural watercourse, usually freshwater, flowing towards an ocean, a lake, a sea, or another river. In a few cases, a river simply flows into the ground or dries up completely before reaching another body of water. Small rivers may also be called by several other names, including...

 in England

England

England is a country that is part of the United Kingdom. It shares land borders with Scotland to the north and Wales to the west; the Irish Sea is to the north west, the Celtic Sea to the south west, with the North Sea to the east and the English Channel to the south separating it from continental...

 which is a tributary of the River Thames

River Thames

The River Thames flows through southern England. It is the longest river entirely in England and the second longest in the United Kingdom. While it is best known because its lower reaches flow through central London, the river flows alongside several other towns and cities, including Oxford,...

. It flows mainly through Hertfordshire

Hertfordshire

Hertfordshire is a ceremonial and non-metropolitan county in the East region of England. The county town is Hertford.The county is one of the Home Counties and lies inland, bordered by Greater London , Buckinghamshire , Bedfordshire , Cambridgeshire and...

 and forms the boundary between the South Bucks

South Bucks

South Bucks is one of four local government districts in the non-metropolitan county of Buckinghamshire, in South East England.The district was formed on 1 April 1974 by the Local Government Act 1972, by the amalgamation of the area of Beaconsfield Urban District with part of Eton Rural District...

 district of Buckinghamshire

Buckinghamshire

Buckinghamshire is a ceremonial and non-metropolitan home county in South East England. The county town is Aylesbury, the largest town in the ceremonial county is Milton Keynes and largest town in the non-metropolitan county is High Wycombe....

 and the London Borough of Hillingdon

London Borough of Hillingdon

The London Borough of Hillingdon is the westernmost borough in Greater London, England. The borough's population was recorded as 243,006 in the 2001 Census. The borough incorporates the former districts of Ruislip-Northwood, Uxbridge, Hayes and Harlington and Yiewsley and West Drayton in the...

. It splits off into several separate branches, some of which rejoin it, and it flows into the River Thames on the reach above Penton Hook Lock

Penton Hook Lock

Penton Hook Lock is a lock on the River Thames in England situated on the northern "Middlesex" bank near Laleham, Surrey.The lock cuts across a large loop or hook in the river, creating Penton Hook Island There is a large marina in the disused gravel pits connected to this loop...

 at Staines

Staines

Staines is a Thames-side town in the Spelthorne borough of Surrey and Greater London Urban Area, as well as the London Commuter Belt of South East England. It is a suburban development within the western bounds of the M25 motorway and located 17 miles west south-west of Charing Cross in...

.
The Colne rises near North Mymms

North Mymms

North Mymms is a civil parish in the English county of Hertfordshire.The village itself has suffered from enclosure. North Mimms Park and Brookmans Park enclose large areas of the parish. Even the parish church stands in the park of North Mimms; in it is a chapel, the burialplace of the Coningsbys...

 in Hertfordshire (although the now dry river bed can be followed further back for about 1.2 miles (1.9 km) to the Water End Swallow holes).