Fleet (The), now an inconsiderable stream, appears to have been formerly of much more importance. Old records and views of London represent it as navigable for boats and barges from Blackfriars, where it falls into the Thames up to Holborn-bridge, then called the Fleet-bridge, where it received the stream called Old-bourn, on the banks of which were scattered the few houses which constituted the hamlet of Old-bourn, from whence the present name of Holborn is derived. The Fleet has its rise at Caen-Wood between Hampstead and Highgate, and formed in its way to the Thames the offensive hollow of Fleet-ditch, which is now arched over for nearly its whole extent. Indeed, as a river, the Fleet has altogether disappeared in London, being now degraded into a mere sewer, or receptacle for filth of all kinds, which emerges into sight near St. Pancras old church.
Source: England and Wales Delineated by Thomas Dugdale assisted by William Burnett; published by Tallis & Co., Green Arbour Court, Old Bailey, 1835.