Digging for gravel and clay along its lower course near Rickmansworth has created a belt of flooded pits below the water table, as established lakes, many of which are well-adapted habitats for wildlife, protected as nature reserves.

The Colne becomes universally so-named after two close subterranean streams converge at a spring, or in very wet weather in the meadows above, all along the east side (Tollgate Road) of North Mymms Park in Hertfordshire.

The source streams are long, multi-source, in final, northern stages, partly underground tributaries: a north-eastern, which can be considered the upper Colne; and the Mimmshall Brook.

[1] A seasonal, intra-parochial stream runs under the seven-building village of North Mymms draining most of the park, which, with adjoining woods is a small square plateau, sloping down to its corners except the south-west.

In the north-west, centred on Tring's northern boundary the Chiltern ridge is most noticeable, a watershed of this basin and that of the River Great Ouse, north and east towards the Wash.

[7] Between Croxley Green, where the River Gade joins the Colne, and Thorney/West Drayton, below the Slough Arm, the river channels thread their way between many large lakes, some of which were once watercress beds, some chalk pits,[8] and some of which were the result of brickmaking, an industry that developed over several miles of the valley after 1800, when the Grand Junction Canal company advertised the presence of good brick earth, discovered during the construction of the canal.

The first is on the north side of the river in Colney Heath Local Nature Reserve, and is a square-section cast iron pillar made by Henry Grissell at his Regents Canal ironworks and erected in 1861 or 1862.

The obelisk records the agreement made between the canal company, John Dickinson who was the owner of papermills at Batchworth, and R. Williams of Moor Park, who was the landowner.