The Colne springs forth from an underground river located in North Mymms Park. From here its influence is reflected in the many locations attributed with its name. Coursing north into Colney Heath, the river slices through Colney Heath Common before altering course to the south west towards London Colney and Broad Colney before its confluence with the River Ver at Colney Street. In so doing it travels along a heavily modified course through former gravel workings (now flooded) of Tyttenhanger Park. Several tributary water courses feed into the Colne from the south, drawing water from sources in Borehamwood and further to the south in the London Borough of Barnet and Stanmore. One of these is shown on maps as 3 water courses with a common name, the Tykes Water. Two of these begin in Abberford Park and Parkfields in Borehamwood. The third passes via a significant lake, formed by a dam, a feature of Aldenham Country Park. Other tributaries flow from Hatfield (Ellenbrook) and Elstree (Mimmshall Brook).
The Upper Colne continues on its way south west alternating between both the urban and rural landscape typical of the south of Hertfordshire. In so doing it passes through historic (Wall Hall) and wildlife rich (Withey Beds Local Nature Reserve) locations. Just downstream the Colne enters the channel of the GU canal. So too does the river Gade and it is at exactly this location that the historic (pre canal) confluence of the Gade and Colne occurred. Whilst the main flow of both is now directed down the canal the original channel still exists, fed by a spill weir, and meanders through Croxley Hall Farm where the confluence of another tributary, the river Chess, is located. The combined flows eventually reach the Colne Valley Park at Rickmansworth.