Template:Infobox lake Template:Location map Aldenham Reservoir is situated in the parish of Elstree and Borehamwood in Hertfordshire, England, about one mile (1.6 km) to the west of the village of Elstree.
Originally, it was Template:Convert in extent [now Template:Convert after repairs to the dam] and was built by the Grand Union Canal Company to control the water levels in the River Colne following the construction of the Grand Union Canal. It was hand dug by French prisoners of war between 1795 and 1797.
It is the source of the stream, Tykes Water, that runs north through Radlett. Nearby is the much larger Hilfield Park Reservoir, a nature reserve owned by Veolia Water.
It now forms the centre piece of the Aldenham Country Park and is used for recreational purposes, fishing and sailing. Aldenham Sailing Club  is based here. Aldenham SC is a dinghy racing club, sailing mostly Laser, Enterprise, and RS300 dinghies.
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Aldenham Sailing Club, Aldenham Reservoir, Elstree, Herts.