The River Ash is a small, shallow river in Surrey, England. Its course of 10 kilometres (6.2 mi) is just outside Greater London. Work has been carried out to re-align, clear and build up a small, Littleton head of water and create two backwaters. One backwater dates to the medieval period; the other to the 1990s. It flows as one of the six distributaries of the River Colne from the south of Staines Moor immediately south of the Staines Bypass eastwards through the rest of the borough of Spelthorne before meeting the River Thames.

It is not navigable to craft and is rich in plant and insects, particularly reeds, diverse sedges (many of which commonly called bulrushes), pond-skaters, amphibians and lepidoptera (moths and butterflies). It enhances the Ash Link Nature Reserve, Studios Walk woodland biodiversity site and two parks. It is recognised as a key ecological feature within its borough.

The river is a distributary of the Colne. It forms the traditional boundary of Staines-upon-Thames, first with Stanwell then with Ashford. It then turns southward and splits Littleton (specifically the Queen Mary Reservoir, then a thin nature reserve by Shepperton Studios) from Laleham. The Ash then resumes eastward. It is the northern limit of diminutive, near-square Shepperton Green — the western third of which was for centuries a southern outcrop (projection) of Littleton, and remains so in the Anglican church system. Eighteen-hole Sunbury Golf Course on high-landscaped former municipal waste landfill then opposes a little of Shepperton across the banks. Then a farm of Green Belt straddles the river partly in outer lands of Sunbury-on-Thames. The river joins the Thames, flowing gently into the Creek – a secondary weirstream of the Thames – facing a long residential island: Wheatley's Ait. It marks two streets' garden ends, many of which have built footbridges. The five urban centres in the borough are well over 500 m away, which has spared the Ash from pollution and supported its biodiversity. Three parks feature the river such as a long walk in Fordbridge Park, Ashford. Canoeists avoid the river when deep enough to canoe due to short barrages, extreme narrows and culverts.

In terms of flow of the distributaries, it is mid-ranked:

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 0.6 miles (0.97 km) to 3 miles (4.8 km) 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 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: