From Wikipedia:

As part of the Tai Tam scheme, the colonial government commenced the construction of Tai Tam Tuk Reservoir in 1912, the project was completed in 1917, cost 2.46 million Hong Kong dollars and had a 1.42 – billion – gallon capacity upon completion. The dam, designed by Daniel Jaffe, was 60 feet tall and 800 feet wide, 12 arches supported by half round granite columns were also built to prop up the Tai Tam road connecting Stanley and Chai Wan. Governor Henry May officially announced the completion of the reservoir on 2nd February 1918 and the reservoir operates until now.

I went there on Sep 25, 2020 after a few weeks of rain in Hong Kong. Unfortunately, water flowing out from the dam had already been much less than 2-3 days before.