The Clarendon Weir is a weir in the Australian state of South Australia, located on the Onkaparinga River in the suburb of Clarendon, about 20 kilometres (12 mi) south of the Adelaide city centre.

[1] Built in 1894–96 as a solution to Adelaide's sewer problems, the weir supplied water to the Adelaide plains to flush the sewers as there simply was not enough water to keep the population healthy.

The water flowed by gravity from the Weir through the tunnel to Happy Valley Reservoir.

Mt Bold Reservoir was added in the 1940s to again supplement Adelaide's water supply with the Clarendon Weir acting as a holding dam down stream.

[citation needed] In the mid-1960s, the wall of the Clarendon Weir, originally constructed with large blocks of Macclesfield marble, was raised from 14 metres (46 ft) to its current height of 14.6 metres (48 ft) .