Thorney Island was formed thousands of years ago by the interaction of the Thames with the Tyburn, one of London's forgotten rivers. The Tyburn still exists today springing, like the Westbourne and the Fleet, from the Hampstead Hills, and although it soon gets lost in Sir Joseph Bazalgette's amazing sewer system it is still possible to chart its subterranean meanderings. From Hampstead it flows down to Regent's Park, crossing the canal in a pipe. It is then carried in another pipe through Baker Street station and then southwards where it flows under Oxford Street through Green Park and under Buckingham Palace where it splits into two. One channel flows goes towards Tothill Street and the other passes very close to the present Victoria Palace theatre before passing under Kings' Scholars' Passage (its exact historic route) then under Vauxhall Bridge Road and along Tachbrook Street. It empties itself into the Thames a few hundred yards west of Vauxhall Bridge where the exit and the cottage of the sluice gate keeper can still be seen.
There are, in effect, two Thorney Islands. The wider one which embraces all the land from the edge of St James Park to the east of Vauxhall Bridge. This is roughly the area which the Thorney Island Society today keeps an eye on. The narrower one - which is the true historic island - starts at the eastern end of Tothill Street where the Tyburn again split into two. One stream turned south and followed the abbey wall down along Great College Street to the Thames near where the medieval monks built the mill - after which Millbank is named - which features in the Norden map of 1593 (see above). The other section headed north along Long Ditch (roughly, today's Storeys Gate) before turning east across today's Whitehall just below Downing Street reaching the Thames near Whitehall Stairs. 
It is a moot point whether the Tyburn exists at all today.Its flow was stemmed as early as 1237 when much of its source water was diverted along a conduit to theCity of London for the benefit of the merchants. WhenBazalgette built the sewers they would be constructed near the course of the river which would then be diverted into the sewer system. The sewer system also has "interceptors" which divert sewage and what is left of the Tyburn eastwards to treatment plants in outer
London long before it reaches the Thames. It is only when there is excess water as a result of storms (which in days of yore would have swelled the Tyburn) that water flows over the interceptors. It still reaches the Thames east of Vauxhall Bridge but the exits at Whitehall and Great College Street have long been blocked off with surplus water being carried east by a large interceptor under the Embankment. So the Tyburn today may just exist in the eye of Romantics except that parts of the route
still exist and some storm water may reach the Thames near Vauxhall that might once have swelled the historic Tyburn.