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 is ordinarily very 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.