Blue Lake (Rotomairewhenua in Māori) 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.Photo credit: flickr – The lake has extremely clear water, and is the clearest natural body of fresh water yet reported. A 2011 study found its visibility ranged from 70 to 80 metres (230 to 260 ft), clearer than the 63 metres (207 ft) measured for Te Waikoropupu Springs, a previous record holder. For comparison, laboratory measurements show distilled water has a visibility of approximately 80 metres (260 ft). Scientists attribute the lake water’s clarity to its passage underground from Lake Constance, which filters out nearly all the particles suspended in the water. Its clarity reveals water’s natural blue-violet colour.
They subsequently organised a scientific study of the lake with involvement of NIWA scientist Mark Gall, an expert in ocean optics instrumentation. Several visits by helicopter (six in all) established that the horizontal visibility in the lake typically ranges from 70–80 metres.
“The theoretical visibility in distilled water is about 80 metres, as estimated from the best available instrumental measurements in the laboratory,” says Dr Davies-Colley. “So Blue Lake is a close approach to optically pure water”.
Lake is characterized by blue-violet hues seen only in the very clearest natural waters. The lake is spring fed from the neighboring glacial Lake Constance, but the water passes through landslide debris that forms a dam between the two lakes.
Photo credits: Project Pressure