Article: Walking is a popular recreational activity in London. Areas that provide for walks include Wimbledon Common, Epping Forest, Hampton Court Park, Hampstead Heath, the eight Royal Parks, canals and disused railway tracks. Access to canals and rivers has improved recently, including the creation of the Thames Path, some 28 miles (45 km) of which is within Greater London, and The Wandle Trail; this runs 12 miles (19 km) through South London along the River Wandle, a tributary of the River Thames. Other long distance paths, linking green spaces, have also been created, including the Capital Ring, the Green Chain Walk, London Outer Orbital Path ("Loop"), Jubilee Walkway, Lea Valley Walk, and the Diana, Princess of Wales Memorial Walk.

Now answer this question: What river is a tributary of the River Thames?
the River Wandle