the Hogsmill by Tolworth Court Bridge under the A240 Kingston Road. From the William Bourne pub to Tolworth Court Bridge, the river serves as a boundary between West Ewell (Surrey) and Chessington (Greater London). Horton Stream The source of Horton Stream is in the centre of Horton Country Park, Epsom. It is 3.3 miles (5.3 km) long and its source is near West Park Hospital beside Horton Lane, Epsom. The stream carries on parallel with Horton Lane as it passes near to West Park Farm and Horton Park Farm. After travelling through Horton Country Park, it heads through Horton Golf Course before crossing Chessington Road, West Ewell. It runs alongside Chessington Close before running through the playing fields of Epsom and Ewell High School. It continues behind Scotts Farm Road before the confluence with the Hogsmill near Ruxley Lane after running under Scotts Farm Road. Tolworth Brook At , Tolworth Brook (or Surbiton Stream) is the longest tributary. Its source is on the edge of Claygate from springs at Grapesome Wood on the west side of the A3, It flows under the A3 to skirt Lovelace Primary School, north under Clayton Road between the withy bed and Oaklands Close, then through rugby playing fields and crosses the A309 Esher bypass going north between allotments and Kelvin Grove. It then passes west of Gladstone and Brook Roads through playing fields, then is culverted from the rear of 1 Herne Road. Its culvert is then visible east of Hook Road at Vale Road North, and after, under Waterside Close and Thornhill Road, to turn east under Cotterill Road. It is the open limit of gardens between Cotterill and Ellerton Roads, until Ditton Road (where culverted). The brook reappears north of pub/restaurant The Oak on Ewell Road. It follows the rear of Beaconsfield Road and then goes under King Charles Road to reappear in Alexandra Recreation Ground, and then flows north-east bisecting Edith Gardens Nature Reserve; east under Raeburn Avenue; north under Meldone Close. It is the central, northward, feature of Berrylands Nature Reserve; under Stirling Walk, it falls east under Elmbridge Avenue to meet the Hogsmill. Water quality The Environment Agency measure 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