Macaskill Lakes

The Stuart Macaskill Lakes (formerly Te Marua Lakes) provide water supply and storage for the greater Wellington urban area. Their main function is to provide clear water for times when the Hutt River, the regions primary water intake, is running too turbid for efficient treatment and distribution. The lakes are sited on abandoned river terraces of the Hutt River, and immediately adjacent to the Wellington Fault.

The scarp of the Wellington Fault is clearly visible on aerial photographs taken prior to construction of the lakes (see photo). It was completely destroyed along with a series of nearby river terraces, when the lakes were constructed.

During construction of the lakes, a 10 m deep trench was excavated across the Wellington Fault. Greywacke bedrock in the trench was crushed and sheared over a zone about 70 m wide; the 75-80° SE dipping fault plane consists of clay fault gouge, up to about 40 cm thick, extending through the river gravels.

Near the Stuart Macaskill Lakes, mains water supply lines cross the Wellington Fault making water supply failure almost inevitable in the event of a Wellington Fault surface rupture. At these crossings, automatic shut-off valves are installed in conjunction with easily repaired smaller-diameter surface by-pass pipes designed to speed restoration of (at least partial) water supply. Despite these and other measures that add robustness to the water supply network, supply of water will be one of the most significant challenges facing the region after a Wellington Fault rupture.

From the lookout, and the images here, try to work out the position of the Wellington Fault trace on the far side of the two big lakes.
Where do you think the water in the lakes actually comes from? Why do you think this site was chosen to store the water supply when it is so close to the fault?
Think about this: If the water supply to Wellington urban area is cut off after an earthquake, where will you be getting your water from?