It is all controlled via sluice gates which are rarely opened. Causes a lot of issues for the fishermen particularly in the summer.
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The Delf flows into Sandwich from a series of springs in the Lydden Valley behind the town of Deal. Some sections are underground now but there are stretches, such as the L-shaped section here, that are open at street level.
The name ‘Delf’ sounds Dutch but it is actually from Old English for ‘ditch’ or ‘dig’. The Delf is not a natural stream; it was diverted by locals in the twelfth century. The water for this innovative system came from marshland outside the town walls.
The Delf was intended to supply the town with clean water but, despite strict laws to maintain it, the results were mixed. Animals got into the water, while beyond Horse Pond Sluice blacksmiths, butchers and tanners used the Delf for their work. The stream was also used to dispose of all kinds of waste.
The Delf waterway at Horse Pond Sluice (top) and alongside New Street Rory Walsh © RGS-IBG Discovering Britain
As a result waterborne diseases were a constant problem in Sandwich. Even so the Delf was used for drinking water until the end of the nineteenth century.”
http://www.open-sandwich.co.uk/places_to_visit/images/sandwich-walk.pdf
