As all members of the society know, it is an island formed when the Tyburn river - now lost among the submerged sewers - met with the Thames forming a delta on which Westminster Abbey and Parliament now reside. It is a seriously historic island.
Or is it ? A new edition of the celebrated book "The Lost Rivers of London", points out that no reliable maps existed before the seventeenth century and adds:  " the island theory still has much to prove". The mainstream (so to speak) view is that when the Tyburn reached what is now Buckingham Palace from its source on the slopes of Hampstead, it bifurcated. One section flowed down to the Thames west of Vauxhall Bridge via Tachbrook Street while the other section flowed from the area of today's Palace along Petty France to the end of Tothill Street where it formed a new delta with one leg reaching the Thames after crossing Whitehall below Downing Street while the other leg meandered around the west of the Abbey wall to empty itself in the Thames at the end of Great College Street where the Abbey Mill was situated and after which Millbank is named.

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However, among rival theories mentioned, is one by archaeologist Tim Tatton-Brown who suggests that in the late eleventh century the monks from the Abbey diverted some of the water from the Tyburn in the Buckingham Palace area to form a man-made channel to the Abbey in order to flush the monks' toilets into the Thames. This was, he says, extended later around the Abbey walls to power the mill at the far end of the Abbey garden.
There is a further complication. In 1238 water was diverted from the Tyburn stream at Stratford Place off Oxford Street and chanelled along a specially built conduit to the City of London to provide it with clean(ish) water. This would obviously have greatly reduced the amount of water flowing into the Thames and would also have helped to drain the marshland between Westminster and Vauxhall Bridge known as Tothill fields. Current archaeological research suggests that there were a number of smaller eyots or islands in addition to Thorney Island, some of which may have disappeared when the tidal level of the Thames rose.
The $64,000 question is whether Thorney Island was formed by the action of the Thames alone or by interaction with the Tyburn. The authors conclude:" It would appear that Thorney Island did exist (Phew!), but there is insufficient evidence as to whether the island was formed as a delta by a prehistoric channel of the Tyburn, or whether it was formed by an accumulation of gravel on a bend in the Thames by the action of that river."

This is an excellent update of Nicholas Barton's original classic book written in conjunction with Stephen Myers, an academic and water engineer who is an expert on the river Walbrook as well. It should not pass without comment that the first edition of the book was published in 1962 and - 54 years later - Nicholas Barton himself was able to present the revised version in March. The Tyburn clearly creates its own form of longevity.