Colne channel the Colne Brook the River Wraysbury which just rejoins the Colne tens of metres from the Thames (so is more properly a later corollary) acting as a distributary. Soil divide and water characteristics Alluvium on gravel (save for London Clay outcrops, such as Harrow on the Hill, Hampstead Heath and the Grim's Ditch (Harrow)) is the soil setting of the north bank, west of the City of London as far as the Chiltern Hills. As to topsoil the river marks a divide between permeable shallow gravelly topsoil for many miles to the north and almost impermeable alluvium to the south. A very thin, agriculturally prosperous alluvial belt by the River Thames to away was caused by deposition from seasonal floods. This prevents for the whole course the rivers joining despite their proximity. As the gravel former terraces of the Thames very gently slope from north to south no abrupt halts to the water table exist no springs arise and only modest interaction with the Thames Basin's water table. The sources are water from the Colne and run-off principally of a fresh water waterworks. Water quality Water quality is ordinarily clean given improvements in the effluent treatment works along the quite populous Colne valley – for a low-gradient river eutrophication and deposition is moderate to low. The river is recognised as a key ecological feature within its Borough. The Environment Agency measures the water quality of the river systems in England. Each is given an overall ecological status, which may be one of five levels: high, good, moderate, poor and bad. There are several components that are used to determine this, including biological status, which looks at the quantity and varieties of invertebrates, angiosperms and fish. Chemical status, which compares the concentrations of various chemicals against known safe concentrations, is rated good or fail. Water quality of the River Ash in 2019: History Ashford gains its name from the river, forming the southern limit of all but its east half (ex-manor demesne and Common). Relevant maps date to medieval times, showing course changes, some clearly man-made. The earliest at close resolution recounts various channels by Shepperton Studios (Littleton Manor demesne), commissioned by the Lord of Shepperton Manor, two miles to the south. This refers to the monks of Westminster, who caused the local widening into heads of water as fish "pond"s. Since 1910 the ex-Metropolitan Water Board's Staines Aqueduct