From the longer Wikipedia page 
The River Neckinger is a subterranean river that rises in Southwark and flows through London to St Saviour's Dock where it enters the River Thames. The river is now totally enclosed and runs underground.
The area where the Neckinger meets the Thames at St Saviour's Dock was historically known as Jacob's Island (now the wealthy area known as Shad Thames). The area was once notoriously squalid and described as "The very capital of cholera" and "The Venice of drains" by the Morning Chronicle of 1849.
In the 17th century convicted pirates were hanged at the mouth of the river (the corpses were placed on display as a deterrent further downstream at Blackwall Point). The name of the river is believed to derive from the term "devil's neckcloth" (i.e. hangman's noose).
The environs are vividly described in Charles Dickens' novel, Oliver Twist as the place that one of Dickens' best-known characters, Bill Sikes, meets a violent death in the mud of St Saviour's Dock.