New Zealand’s Blue Lake—the clearest lake in the world

Blue Lake (Rotomairewhenua in Māori) is a small fresh water lake in Nelson Lakes National Park, in the northern reaches of New Zealand's Southern Alps. In a study conducted in 2011, researchers of the National Institute of Water and Atmospheric Research (NIWA) have found that the lake has extreme visual clarity of up to 80 m, which is considered almost as ‘optically clear’ as distilled water. The visibility of Blue Lake even surpasses that of the renowned Te Waikoropupu Springs in Golden Bay, which has a visibility of 63 m. Blue Lake is characterised by blue-violet hues seen only in the very clearest natural waters. The lake is spring fed from the neighbouring glacial Lake Constance, but the water passes through landslide debris that forms a dam between the two lakes. The natural dam filters out nearly all the particles suspended in the water giving the lake the most intense natural blue-violet colour.

Nelson-based NIWA hydrologist Rob Merrilees first recognised that Blue Lake might be optically outstanding, having observed on tramping trips that this water body appeared broadly similar to Te Waikoropupu. He mentioned his suspicions to NIWA aquatic optics specialist, Dr Rob Davies-Colley, who had led the original work on Te Waikoropupu Springs. On a preliminary tramping visit to Blue Lake in March 2009, the two Robs were surprised to find that the visibility of Blue Lake exceeded that of Te Waikoropupu.

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 m. "The theoretical visibility in distilled water is about 80 m, 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.