The Fleet river rises as two springs feeding the ponds of Hampstead and Highgate and flows down through Belsize Park and Kentish Town before merging just east of Camden. The river then meanders passed King's Cross, formerly known as Battle Bridge. In Roman times the river here was 20 metres wide, and the bridge was the scene of a major battle between Boudica and the occupiers in AD60.

The Farringdon Road then maps its path, passing by Smithfield ( Cowcross Street is so named because cattle forded the river here on the way to market ) and close to St. Paul's before flowing into the Thames at Blackfriars. Fleet is derived from flēot, the anglo-saxon word for tidal inlet, and before human intervention the marshy basin where the Fleet met the Thames was over 100 metres wide.

The river Fleet today is not visible between the ponds of NW3 and N6 and the Thames, but in certain places en route you can hear it..