Entrance to the Fleet River c. 1750 by Unknown artist.

Entrance to the Fleet River c. 1750 by Unknown artist.

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From Wikipedia

The Fleet rises on Hampstead Heath as two sources, which flow on the surface as the Hampstead Ponds and the Highgate Ponds. Then they go underground, pass under Kentish Town, join in Camden Town, and flow onwards towards St Pancras Old Church, which was sited on the river's banks. From there it passed in a sinuous course which is responsible for the unusual building line adjacent to King's Cross station; the German Gymnasium faced the river banks, and the curve of the Great Northern Hotel follows the river, which passes alongside it.

King's Cross was originally named Battle Bridge, referring to an ancient bridge over the Fleet where Boudica's army is said to have fought an important battle against the Romans; the name was changed in the 19th century to refer to a statue of George IV and quickly replaced by the 'lighthouse' building that still stands today. From there, it heads down King's Cross Road, from where the valley slope can be seen in the surrounding streets, and into Clerkenwell; the comparatively steep valley is responsible for the viaduct bridges which carry local roads over the valley floor.

The river then flows down Farringdon Road and Farringdon Street, where the valley broadens out, and straightens, and joins the Thames beneath Blackfriars Bridge.