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. Their main sources are : in western Northaw, Brookmans Park, Hadley, London and Borehamwood, draining much of those places. 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.