Dams

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 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.

• 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.

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.

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 Waitākere Ranges cover 162 hectares and normally provide 20 per cent of Auckand's water supply.

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.

The dams that recently reached 90+ per cent capacity or have been spilling are Upper Nihotupu and Waitākere. 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 we have been maximising its production.

For more information on the size and capacity of individual dams, click here.

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. Recently, 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. In order to reduce the amount of spillage, our team has been maximising production from Waitākere Dam.

Our dams in the Hūnuas aren't full because Auckland is still making up for a long-term lack of rainfall. Our dams in the Hūnua Ranges are our biggest - combined they cover 534 hectares and provide 60 per cent of Auckland’s water. In comparison, our dams in the Waitākere Ranges fill up quicker as they are much smaller in size/capacity, covering 162 hectares and providing 20 per cent of Auckand's water.

Between November 2019 and May 2020, Auckland received 40 per cent less rainfall than normal, making it the region’s worst drought on record. Despite recent rainfall, our total water storage is well below where it should be for this time of year and heading into the dry summer months.

Hays Creek Dam is tiny compared to our biggest dams Mangatangi and Upper Mangatawhiri. Hays Creek Dam 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. It was supplying only a fraction of Auckland's drinking water and was expensive to run.

Hays Creek Dam will be reconnected to the metropolitan supply network in December 2020.