Aside from holding sufficient supplies of water, Auckland’s 12 dams in the Waitakere and Hunua ranges serve another purpose: when water is stored in a lake, the quality improves. This is because sediment particles settle on the lakebed and bacteria levels reduce. Dams are also useful for storing storm run-off which can help reduce the effect of floods.
Dam location
It makes sense to locate dams in high rainfall areas close to cities. The Hunua and Waitakere ranges have a high elevation and receive around 1.8 metres of rain annually, about 50 percent more than central Auckland gets. The high location of the dams also makes it easy for water to flow to the city by gravity.
Dam structure
Auckland’s dams are constructed with either compacted earth or concrete. The earlier dams in the Waitakeres were made of concrete while the later, post-war dams were built with earth and rock.
Dams in the Hunua Ranges
There are five earth dams in the Hunua Ranges which supply around 57 percent of Auckland’s water. Each was constructed between 1951 and 1977 on the headwaters of the river it is named after, apart from Cosseys, which is on the headwater of the Wairoa River. The water from each dam is piped to the Ardmore Water Treatment Plant. It is then stored and transported through pipes, which are up to 1.9 metres in diameter, into reservoirs nearer the city.
Mangatangi Dam
Completed: 1977
Lake area: 185 hectares
Capacity: 35.3 gigalitres
Mangatawhiri Dam
Completed: 1965
Lake area: 128.5 hectares
Capacity: 16.2 gigalitres
Cosseys Dam
Completed: 1955
Lake area: 123 hectares
Capacity: 14.03 gigalitres
Wairoa Dam
Completed: 1975
Lake area: 98 hectares
Capacity: 11.6 gigalitres
Hays Creek Dam
Completed: 1967
Lake area: 18.2 hectares
Capacity: 1.1 gigalitres
Dams in the Waitakere Ranges
The Waitakere Ranges is home to five dams which supply around 26 percent of Auckland's water. The dams were constructed between 1907 and 1971, three of concrete and two of earth. Each dam is named after the stream that feeds into its reservoir. Raw water from these dams is piped to treatment plants in Huia, Titirangi and Swanson.
Waitakere Dam
Completed: 1910
Lake area 25.1 hectares
Capacity: 1.76 gigalitres
Upper Nihotupu Dam
Completed: 1923
Lake area: 12.5 hectares
Capacity: 2.2 gigalitres
Lower Nihotupu Dam
Completed: 1948
Lake area: 52.9 hectares
Capacity: 4.6 gigalitres
Upper Huia Dam
Completed: 1929
Lake area: 18.9 hectares
Capacity: 2.2 gigalitres
Lower Huia Dam
Completed: 1971
Lake area: 50.3 hectares
Capacity 6.4 gigalitres