The River Fleet is the largest of London's subterranean rivers, all of which today contain foul water for treatment. It has been used as a culverted sewer since the development of Joseph Bazalgette's London sewer system in the mid-19th century with the water being treated at Beckton Sewage Treatment Works. Its headwaters are two streams on Hampstead Heath, each of which was dammed into a series of ponds—the Hampstead Ponds and the Highgate Ponds—in the 18th century. At the southern edge of Hampstead Heath these descend underground as sewers and join in Camden Town. The waters flow 4 miles (6 km) from the ponds.