Built between 1892 and 1897 at a cost of it was the third dam constructed in South Australia as a supplement to the
Thorndon Park Reservoir (built 1860) and the
Hope Valley Reservoir (built 1872). The original Happy Valley township, school and cemetery were completely flooded by the impounded reservoir, requiring their relocation. The township was moved to the east while the cemetery, which is still in use today, was moved to the west and relocated alongside the base of the dam wall. The school, originally located on Candy road, was relocated south to on Red Hill Road (later renamed Education road) which was donated by local farmer, Harry Mason. While some students attended O'Halloran Hill or Clarendon schools for the 18 months that the Happy Valley school was closed, some did not attend any school until it was re-opened on 26 September 1898. The school closed in December 1979 and re-opened on a new site on the other side of the road directly opposite. The
earth-filled dam wall is high and long. When full, the reservoir has capacity of and covers a surface area of , drawn from a
catchment area of . The
uncontrolled spillway has a flow capacity of . The reservoir acts as a 'holding pond' for water directed to it from the
Clarendon Weir via a tunnel. The diameter tunnel was bored simultaneously from both ends and when meeting had a deviation of . The tunnel's deepest point underground is where it passes through a hill. On 7 August 1896 the tunnel's inlet valve was opened by the
Governor of South Australia,
Sir Thomas Fowell Buxton and the reservoir began filling.
Continuing use Located from the Adelaide central business district, the reservoir is now largely enveloped by the city's southern suburbs, of which the relocated Happy Valley village is now one. Although the reservoir is relatively small in capacity; holding only and is dwarfed by
Mount Bold Reservoir which is at least four times larger, Water from the dam was originally supplied to Adelaide through a tunnel under Black Road. In the early 1960s, the original intake tunnel from the Clarendon Weir was increased in size to allow access by maintenance vehicles, and a second outlet tunnel was constructed under
South Road. In 1986 this new tunnel became the sole outlet for the reservoir when the original outlet was abandoned. At the same time the
Thorndon Park Reservoir was decommissioned and reestablished as a recreational park.
Upgrades Between 2002 and 2004, the reservoir underwent a major renovation as part of an rehabilitation project aimed at enhancing the reservoir to meet guidelines of best practice for dam management at both international and national levels. With the lowering of the water level during renovations exposing the original Happy Valley township for the first time,
archaeologists took the opportunity to excavate the site. Despite the township being entirely intact and undamaged when flooded in 1896, very little was found apart from scattered bricks and the foundations of several buildings, of which only the post office was identified. In 2020, a further major renovation occurred, when the majority of the remaining pine plantations were cleared to allow the construction of a ground-mounted
solar panel array, designed to supply the water treatment facility entirely with
green energy, reducing operating costs. ==Recreational use==