Rotomairewhenua / Blue Lake
Interesting facts
- The highest single reading of underwater visibility in the lake reached 81.4 metres, around the theoretical limit for perfectly pure water.
- Its water is clearer than almost all seawater; only patches of the South Pacific near Easter Island and Tahiti are known to rival it.
- Scientists measured the clarity horizontally, sighting a black disc one metre across through the water rather than looking down from the surface.
- In an early test in 2009 the disc was still visible at about 65 metres, measured with a 50-metre tape lengthened with string and boot laces.
- Its exceptional clarity was first noticed in 2001 by a tramper who later co-authored the scientific study that confirmed the record.
- The whole lake, about 180,000 cubic metres, is flushed by spring water in under a day, which is why it clears within days after a muddy flood.
- Out of respect for its sacred status, Ngāti Apa ask that no one washes themselves, clothing or dishes in the lake or uses soap near it.
Why it is worth it
Deep in the mountains of the northern South Island, a small lake lies at the head of a remote valley, and its water is so transparent that the boulders and plants on its bed seem to float in air. Rotomairewhenua / Blue Lake holds the clearest natural fresh water ever measured: a study published in 2011 recorded underwater visibility of 70 to 80 metres, comparable to laboratory measurements for distilled water and well beyond the previous record set at Te Waikoropupū Springs.
The lake sits at about 1,180 metres above sea level in Nelson Lakes National Park, at the northern end of the Southern Alps. It is small, around 2.5 hectares, with two arms of roughly 200 metres each that give it a boomerang shape, and only about 7 to 8.5 metres deep. Its water stays cold all year, between about 5 and 8 °C. The secret of its clarity lies upstream. Blue Lake is fed by the uppermost reach of the West Sabine River, which in turn emerges from seepage through a huge old landslide that dams the much larger Lake Constance above it. Passing through the rubble, the water is filtered of almost every suspended particle, and what reaches Blue Lake shows the true colour of pure water, a deep blue shading into violet. Its outflow becomes the West Sabine, part of the Buller River system.
For the local iwi, Ngāti Apa ki te Rā Tō, the lake is tapu, sacred. It was traditionally used in rites to cleanse the bones of the dead and release their spirits for the journey to Hawaiki, with Blue Lake used for men and Lake Constance for women. The Māori name means "the lake of peaceful lands". Under the iwi's Treaty of Waitangi settlement of 2010 the lake was vested in the tribe and then returned to the Crown as a gift, and in 2014 its official name became the dual Rotomairewhenua / Blue Lake.
Getting there
The lake can only be reached on foot. It lies in the upper West Sabine valley, usually visited as a side trip from the Travers–Sabine Circuit, a multi-day tramping route that starts and ends at the village of St Arnaud, about 32 km away in a straight line. Richmond is about 92 km away, and Nelson Airport (NSN) about 97 km.
On these lists
From the encyclopaedia
Rotomairewhenua / Blue Lake is a small lake in Nelson Lakes National Park, in the northern reaches of New Zealand's Southern Alps. Sacred to local Māori, it has the clearest natural fresh water in the world.
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