How glaciers turn lakes turquoise
Four large lakes on New Zealand's South Island stand out for their distinctive turquoise colour. As seen in this natural-colour satellite image, Lake Pukaki, Lake Tekapo, Lake Ohau, and Lake Benmore all have remarkably light tones in comparison to the dark blue waters of Lake Wanaka and Lake Hāwea to the southwest.
The reason for the difference? The turquoise lakes are fed by rivers where large, active glaciers are pushing through rocky valleys upstream. The glaciers function like bulldozers, grinding away and pulverizing rocks along valley floors and walls. The process produces a fine-grained powder of silt and clay—glacial flour—that is picked up by meltwater streams. Since the particles are so fine, they are slow to sink to the bottom, remaining suspended in the water column instead.
When sunlight hits the water, these particles absorb the shortest wavelengths: the purples and indigos. Meanwhile, the water absorbs the longer wavelength reds, oranges, and yellows. That leaves mainly blues and greens to get scattered back to our eyes.
When the Operational Land Imager (OLI) on Landsat 8 acquired the images, water rich with glacial flour was flowing from the Godley River into the north end of Lake Tekapo. Meltwater from the Classen, Gray, and Godley glaciers all drain into the river. Meltwater from the Tasman and Hooker glaciers flows through the Tasman River into Lake Pukaki.
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Credit: NASA Earth Observatory images by Lauren Dauphin, using Landsat data from the U.S. Geological Survey and MODIS data from NASA EOSDIS/LANCE and GIBS/Worldview.