We don't want to go very deeply into this subject but according to this report: Holborn and Finsbury Divisions of Sewers. Report of the Surveyor [J. Roe] to the Court held 27th January 1843 ... By City of London (England). Commissioners of Sewers, in the middle of the 19th century there were, let's say, 'deposits' that needed to be dealt with. A paragraph of the report will suffice:
"Several houses by the side of the open part of the River Fleet, at Kentish Town, have drains from their privies emptying into the Sewer; these drains being nearly filled with soil, which exuded, presented a most filthy spectacle along the side of the open Sewer: a temporary flushing gate was constructed to pen up the water, so as completely to fill the drains; when the gate was opened, the sudden flow of the water brought out with it the soil with which the drains were nearly filled, and left them almost in as clean a state as when first built; and the soil and filth from the open Sewer cleared away."
Finsbury and Holborn were administrative districts, two divisions of the four in the Hundred of Ossulstone, in Middlesex.
