The River Ash is a small, shallow river in Surrey, England and its course of 10 km or 6 miles is just outside Greater London. Work has been carried out to straighten, sluice control, dredge and hem in sections over centuries. It flows as one of the seven present distributaries of the River Colne from just south of Staines Moor by the M25 motorway eastwards through the rest of the borough of Spelthorne before meeting the River Thames.