Rain falls on the North Downs, filters through the chalk and emerges on the spring line at the Wandle's two sources, both at about 115 ft (38 yd, 35 m) above sea level. These are a pond in Waddon Ponds beside Mill Lane, Croydon; and a secondary source at Carshalton Ponds. It passes through the London Boroughs of Croydon, Sutton, Lambeth, Merton, and Wandsworth to join the River Thames. The names of the river and of Wandsworth are thought to have derived from the Old English "Wendlesworth" meaning "Wendle's Settlement".
In prehistoric times, the river probably flowed from the Surrey Weald northward across the North Downs through the Merstham Gap. In more recent times, rainwater falling on the Down percolates through the chalk and reappears as springs in central Croydon, Beddington, and Carshalton. The occasional stream, known as the Bourne, which runs through the Caterham and Smitham Bottom (Coulsdon) valleys is a source of the River Wandle but only surfaces after heavy rainfall. A series of ditches and culverts channels the water from Purley to Croydon.
For many centuries the River Wandle rose from a spring near the present Swan & Sugarloaf pub, on Brighton Road and flowed through the Haling area. It then ran northwards along Southbridge Road and by the time it reached Old Town it was 20 feet wide and began to divide into smaller channels. The grounds of the Old Palace and Scarbrook Hill had several springs, ponds, streams and canals where fish swam, especially trout. However, as Croydon's population grew, the Old Town streams became little better than open sewers and were filled in or culverted from 1840 after outbreaks of typhoid and cholera.
The river then flowed through Pitlake (meaning 'stream in a hollow') and on through two marshy fields - Froggs Mead and Stubbs Mead, which became Wandle Park in 1890. Local springs were used to form a boating lake in the Park, but frequent drying up problems led to the lake being filled in. The Wandle now continues underground, through where the Gas Works used to stand, under the Purley Way road and into Waddon Ponds.
A tributary starts in Thornton Heath as the Norbury Brook, becomes the River Graveney and joins the Wandle near Summerstown. For part of its length it forms the boundary between the London Boroughs of Croydon and Lambeth and, further downstream, the border between Merton and Wandsworth - from 1900 to 1963 the official border between Surrey and London.
'Village' names in the Wandle basin include: Croydon, Waddon, Beddington, Wallington, Carshalton, Hackbridge, Mitcham, Ravensbury, St Helier, Morden, Wimbledon, Merton Abbey, Colliers Wood, Summerstown, and Wandsworth.
The river has been well-used since Roman times and was heavily industrialised in the 17th and 18th century (the industrial revolution), at one point being one of the most polluted rivers in England. The main industries of the period were tobacco and textiles.
The Liberty print works and Merton Board Mills once dominated the riverscape in what is now the London Borough of Merton. The concentration of heavy industry in this area resulted in the stretch of the river running between Windsor Avenue and Colliers Wood High Street being diverted during the 18th century. The original course of the river still runs underground beneath Liberty Avenue, surfacing at Runnymede as the Pickle Ditch and rejoining the modern river outside Sainsbury's. Few local residents realise that the stretch of the river running past Merton Abbey Mills craft village and in front of Sainsbury's is actually man-made.
Subsequent cleanups of the river have led to a dramatic improvement in water quality leading to a return of the river's once famous Brown Trout. This improvement in water quality has also seen other fish thrive with stocks of Chub, Roach and Perch all flourishing once again with the most popular angling spots situated on the river at Colliers Wood. Around the 17th September 2007, chemicals from a water treatment works operated by Thames Water, were accidentally flushed into the Wandle in the Mill Green area, with reports of around 2,000 fish of various species being killed. The company assumed responsibility for the mistake, and said they were "mortified" by the incident. A blunder at Beddington Sewage Treatment Works was identified as the cause. Sodium hydrochloride was being used to clean its tertiary treatment screens, but instead of being circulated back through the treatment works, it was accidentally discharged into the river. The company immediately offered to meet with the local angling clubs and the Wandle Trust to discuss restocking and long term support for the work of the Trust.
The river is heavily managed with artificial channels, runoff ditches and subterranean stretches.