He kōrero mō ā tātou pāpuni
About our dams
Dam water comes from catchments high in the ranges
A lot of Auckland’s water comes from dams (manmade water storage lakes) in the Hūnua and Waitākere ranges.
Our aim is to capture as much water as possible so that Aucklanders can use water on demand, and to have water available for firefighting if required.
Dam information
Dam information
Where does the water come from?The water comes from protected catchments that are mostly native bush, high up in the ranges. Streams and tributaries feed water to fill the reservoirs created by these dams.
Catchments
The dams only capture water from the upper parts of the catchments. Some of our dams have extensive areas below them which contribute to the total stream or river flows. Occasionally there are extensive storms that do not pass over the part of the catchment above the dam.
The water comes from protected catchments that are mostly native bush, high up in the ranges. Streams and tributaries feed water to fill the reservoirs created by these dams.
Catchments
The dams only capture water from the upper parts of the catchments. Some of our dams have extensive areas below them which contribute to the total stream or river flows. Occasionally there are extensive storms that do not pass over the part of the catchment above the dam.
What is the difference between water storage dams and hydro-electric dams?- The water stored for hydro-electric power passes through a dam to generate power. This water is then discharged to a downstream water course. Hydro-electric dams also have ‘release gates’ to control reservoir levels.
- Water from our reservoirs flows to a water treatment plant, is treated to ensure it is safe to drink, and then carried to consumers. Our dams do not have release gates.
- The water stored for hydro-electric power passes through a dam to generate power. This water is then discharged to a downstream water course. Hydro-electric dams also have ‘release gates’ to control reservoir levels.
- Water from our reservoirs flows to a water treatment plant, is treated to ensure it is safe to drink, and then carried to consumers. Our dams do not have release gates.
Benefits from dams- When water is stored in a lake, the quality improves. Sediment settles on the lakebed, and the level of bacteria drops.
- Capturing water during heavy rainfall minimises the likelihood and impact of downstream flooding. When the dams do reach capacity, they are designed to spill into an engineered spillway. When this happens, the volume of water spilling reflects the volume of natural rain falling at that time.
- To make sure our dams do not adversely affect the downstream environment, we release a small flow of water from each dam to maintain flora and fauna habitats. This ‘compensation flow’ supports the environment when our dams are not full and spilling.
- Free-discharge valves are required for dam safety. We operate the valves periodically to ensure they work correctly, for environmental purposes and to meet resource consent requirements.
- When water is stored in a lake, the quality improves. Sediment settles on the lakebed, and the level of bacteria drops.
- Capturing water during heavy rainfall minimises the likelihood and impact of downstream flooding. When the dams do reach capacity, they are designed to spill into an engineered spillway. When this happens, the volume of water spilling reflects the volume of natural rain falling at that time.
- To make sure our dams do not adversely affect the downstream environment, we release a small flow of water from each dam to maintain flora and fauna habitats. This ‘compensation flow’ supports the environment when our dams are not full and spilling.
- Free-discharge valves are required for dam safety. We operate the valves periodically to ensure they work correctly, for environmental purposes and to meet resource consent requirements.
Living downstream of damsDuring heavy rainfall, the dam will provide attenuation to minimise the likelihood and impact of downstream flooding. However, the dam will spill and excess water flows downstream through a spillway as the dam reaches full supply level.
If you live downstream of or near to a dam, here are some tips:
- Keep informed on weather reports, and on weather warnings.
- Have a 'Get Ready Get Thru' plan, with an emergency plan and kit.
- Check our dam storage levels.
- Understand how Auckland Council and the Local Disaster Management Group manage emergency situations in your area.
See also: Rosedale Dam safety information brochure
During heavy rainfall, the dam will provide attenuation to minimise the likelihood and impact of downstream flooding. However, the dam will spill and excess water flows downstream through a spillway as the dam reaches full supply level.
If you live downstream of or near to a dam, here are some tips:
- Keep informed on weather reports, and on weather warnings.
- Have a 'Get Ready Get Thru' plan, with an emergency plan and kit.
- Check our dam storage levels.
- Understand how Auckland Council and the Local Disaster Management Group manage emergency situations in your area.
See also: Rosedale Dam safety information brochure
Facts about our dams
We operate 12 water supply lakes, five of them located northwest of Auckland in the Waitākere Ranges, two near Helensville, four to the southeast in the Hūnua Ranges, and one east of Papakura. Combined the lakes are capable of storing 95.5 million cubic metres of water.
- Normally the dams supply about 80 per cent of Auckland’s water.
- Because the dams are located at high elevations, water flows easily to the city by gravity.
- Earlier dams in the Waitākeres were made of concrete. After World War II they were built with earth and rock.
- The Hūnua and Waitākere ranges receive about 1.8 metres of rain annually – about 50 per cent more than central Auckland.
Dams in the Hūnua Ranges
The water is piped to Ardmore Water Treatment Plant. It is then stored and carried through pipes, which are up to 1.9 metres in diameter, into reservoirs nearer the city.
- 5 earth dams
- Supply: Normally about 60 per cent of Auckland’s water
- Built: 1951-1977
- Each dam is named after the river that feeds into its reservoir.
Mangatangi Dam
Completed: 1977 Lake area: 185 hectares Capacity: 35.3 gigalitres
Upper 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 Waitākere Ranges
Raw water from these dams is piped to treatment plants in Huia, Titirangi and Swanson.
- 5 dams - 3 concrete, 2 earth
- Supply: Normally about 20 per cent of Auckland's water.
- Built: 1907-1971
- Each dam is named after the stream that feeds into its reservoir.
Waitākere Dam and Waitākere Saddle 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: 21.4 hectares Capacity: 2.2 gigalitres alt=" " Lower Huia Dam Completed: 1971 Lake area: 50.3 hectares Capacity: 6.4 gigalitres
Lower Huia Dam
Completed: 1971 Lake area: 50.3 hectares Capacity: 6.4 gigalitres
Mangakura Dam and Sandhills Weir
Raw water from these two sources is transported to Helensville Water Treatment Plant which supplies treated water to Helensville and Parakai.
Our water supply dams don't contribute to flooding
There is a common misconception that our dams contribute to flooding – in fact, they do quite the opposite. When they do fill up and spill, the water flow rate over the spillway is slightly less than what would be flowing in the stream if the dam wasn’t there. This is because the presence of the dam delays and suppresses peak flows in heavy rainfall.
We don’t have any control over the rate at which water flows from our dams to the stream below when the dams are spilling, as this is driven by rainfall. We do not have ‘release gates’ like hydro-electric dams.
Our dams are operated under various resource consents granted by either Auckland Council or Waikato Regional Council. One of the consent requirements is to release a small flow of water from our dams to protect the downstream flora and fauna. We call this a ‘compensation flow’. The volume of this flow is very low – designed to keep the stream healthy, and maintain the stream habitat, by providing a base flow when our dams are not spilling.
We are also required to carry out periodic flushing operations at our dams in summer using a ‘free discharge’ valve. This again promotes the health of the downstream environment. Typically, we run these flushing flows for about three hours, and the free discharge valve is only partially opened. The flow rate released during these operations is well below what would be considered a ‘flood’ flow.
Releasing water for critical maintenance
We are also authorised to release water from our dams for operational and maintenance purposes, for example to undertake renewal work on the dam that requires a lower water level to get the job done safely.
When we need to draw down lake levels for dam maintenance, we release water slowly over several weeks. In line with dam safety guidelines, we test our free discharge valves once a year by opening them fully for a short period.
When we need to release water for testing or maintenance, we follow dam safety guidelines and the conditions of the dam’s resource consent, which include considerations like the maximum drawdown rate. We also notify the council and any stakeholders downstream.
Frequently asked questions about our dams
Frequently asked questions about our dams
How much water do the Hūnua dams provide?Our dams in the Hūnua Ranges are by far the largest. Combined they cover 534 hectares and normally provide 60 per cent of Auckland’s water.
Our dams in the Hūnua Ranges are by far the largest. Combined they cover 534 hectares and normally provide 60 per cent of Auckland’s water.
How much water do the Waitākere dams provide?Our dams in the Waitākere Ranges cover 162 hectares and normally provide 20 per cent of Auckand's water supply.
Our dams in the Waitākere Ranges cover 162 hectares and normally provide 20 per cent of Auckand's water supply.
How do our dams compare in size?Mangatangi is our largest dam. Completed in 1977, it has a lake area of 185 hectares and capacity of 35.3 gigalitres. Upper Mangatawhiri is our second largest dam. It has a lake area of 128.5 hectares and capacity of 16.2 gigalitres.
In terms of capacity, Upper Nihotupu Dam holds 2.2 gigalitres and spills into Lower Nihotupu Dam. Waitākere Dam holds 1.76 gigalitres and doesn't spill into another dam, so when it's full we maximise its production.
See the current storage of each individual dam here.
Mangatangi is our largest dam. Completed in 1977, it has a lake area of 185 hectares and capacity of 35.3 gigalitres. Upper Mangatawhiri is our second largest dam. It has a lake area of 128.5 hectares and capacity of 16.2 gigalitres.
In terms of capacity, Upper Nihotupu Dam holds 2.2 gigalitres and spills into Lower Nihotupu Dam. Waitākere Dam holds 1.76 gigalitres and doesn't spill into another dam, so when it's full we maximise its production.
See the current storage of each individual dam here.
Why is Waitākere Dam typically one of the first to spill?Waitakere Dam is the highest elevated dam, and like Hays Creek Dam it is tiny compared to our biggest dams which are located in the Hūnua Ranges. It spans 25.1 hectares, has a capacity of 1.76 gigalitres and is the smallest dam in the Waitākere Ranges. In 2020, it reached 100 per cent capacity and was spilling. Its size along with the fact it has received more rainfall than other catchment areas are key factors in causing it to spill. When full, in order to reduce the amount of spillage our team maximises production from Waitākere Dam.
Waitakere Dam is the highest elevated dam, and like Hays Creek Dam it is tiny compared to our biggest dams which are located in the Hūnua Ranges. It spans 25.1 hectares, has a capacity of 1.76 gigalitres and is the smallest dam in the Waitākere Ranges. In 2020, it reached 100 per cent capacity and was spilling. Its size along with the fact it has received more rainfall than other catchment areas are key factors in causing it to spill. When full, in order to reduce the amount of spillage our team maximises production from Waitākere Dam.
How much does Hays Creek Dam store?Hays Creek Dam is tiny compared to our biggest dams Mangatangi and Upper Mangatawhiri. It spans 18.2 hectares and has a capacity of only 1.1 gigalitres. Interestingly, unlike our other catchment areas, it is not a protected catchment. This dam was taken out of service several years ago as it was supplying only a fraction of Auckland's drinking water and was expensive to run. It was reconnected to the metropolitan supply in February 2021, boosting Auckland’s capacity by six million litres a day and by up to 12 million litres a day by August 2023.
Hays Creek Dam is tiny compared to our biggest dams Mangatangi and Upper Mangatawhiri. It spans 18.2 hectares and has a capacity of only 1.1 gigalitres. Interestingly, unlike our other catchment areas, it is not a protected catchment. This dam was taken out of service several years ago as it was supplying only a fraction of Auckland's drinking water and was expensive to run. It was reconnected to the metropolitan supply in February 2021, boosting Auckland’s capacity by six million litres a day and by up to 12 million litres a day by August 2023.
Why can’t you strategically keep your dams low and prevent them from spilling?Auckland’s water supply dams aren’t designed for the rapid drawdown of their water levels and are not suitable for flood management purposes. If they did have the infrastructure to do this, we would lose a significant amount of the city’s water storage capacity and reduce the water available during a drought, which would bring forward the need to invest in new water sources.
Auckland’s water supply dams aren’t designed for the rapid drawdown of their water levels and are not suitable for flood management purposes. If they did have the infrastructure to do this, we would lose a significant amount of the city’s water storage capacity and reduce the water available during a drought, which would bring forward the need to invest in new water sources.