Nelsons Blue Lake – The Clearest Freshwater ever Reported

Nelsons Blue Lake at the top of New Zealand's South Island has some of the clearest water in the world, a study has found. The water is considered almost as "optically clear" as distilled water. Niwa aquatic optics specialist Rob Davies-Colley said it was highly significant that the clarity of the water in the lake was close to pure water.

Blue Lake (Rangimairewhenua in Māori) is a lake in Nelson Lakes National Park, in the northern reaches of New Zealand's Southern Alps and is sacred to local Māori, it now has been deemed to have the clearest natural fresh water in the world. Research by the National Institute of Water and Atmospheric Research shows that the visibility in the Blue Lake is up to 80 metres (262 feet). The water is considered almost as "optically clear" as distilled water. It surpasses the renowned Te Waikoropupu, or Pupu, Springs in Golden Bay which is relatively close by, where visibility of up to 63m (206 ft) has been reported (Image at bottom). Water visibility was considered outstanding when it exceeded about 40m (131 ft), Niwa said.   "Nelsons Blue Lake is characterised by blue-violet hues seen only in the very clearest natural waters and it is highly unusual in its geological and hydrological setting, being spring-fed from neighbouring glacial Lake Constance. Almost all suspended particles appear to be filtered from the water as it passes through landslide debris that forms a dam between the two lakes – which probably accounts for its extreme visual clarity." Reports Niwa scientist Mark Gall. Researchers suspended a black disc, a metre in diameter, under a buoy and then paddled away in a boat while looking at the disc - visibility was measured when the disc disappeared. Niwa aquatic optics specialist Rob Davies-Colley said it was highly significant that the clarity of the water in the lake was close to pure water. Background:

• Nelsons Blue Lake is a side trip off the popular Travers Sabine Circuit multi day tramp.
• The Blue Lake Hut receives in excess of 700 trampers a year, mainly through the summer months.
• Numbers will be monitored to see if visitation increases.
• The clarity of the lake will remain monitored to establish if there are any changes in the next year or two.
• The waters of Blue Lake/ Rangimairewhenua are sacred to Ngāti Apa ki te Rā Tō iwi.
• The significance to them was acknowledged in their Treaty of Waitangi settlement signed in 2010.
• The Department of Conservation would ask that the waters are respected and that people don't wash themselves or their clothes or dishes in the lake.

Above: Te Waikoropupu, or Pupu, Springs in Golden Bay. Visibility of up to 63m (206 ft)

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