Glaciers work kind of like gigantic sheets of sandpaper. You know how, when you rub the coarse side of sandpaper over a wooden surface, you create a lot of fine sawdust? Well, glaciers do pretty much the same thing, except they work on mountain rock rather than wood. Glacial ice moves very slowly downhill. While it moves, bits of rock and gravel get stuck between the ice and the mountain, forming a coarse surface similar to the sand on sandpaper. The rocks grind together, creating the glacier equivalent of sawdust. Geologists call this dust “rock flour,” and as glaciers melt during spring and summer, they transport it down into mountain lakes.
Here’s where things get colorful. The rock flour is so fine that it doesn’t sink to the bottom of the lake. Instead, it remains suspended throughout the water. When sunlight hits, the water absorbs the long wave colors of the spectrum: the reds, oranges, and yellows. At the same time, the rock flour absorbs some of the shortest light waves, the purples and indigos, then scatters the remaining light back to our eyes. Thanks to these processes, the reflected light is mostly green, with a dash of blue. And voilà: turquoise water!
Each glacier produces its own flavor of rock flour and as such, no two rivers or lakes sport the same hue. The intensity of the color of the water is dependent on several factors: the amount of glacial runoff, the composition of the rock flour, how much rain the region has seen recently (a lot of rain can dilute the runoff), and the angle at which the water is viewed. I highly recommend taking a further look at some of my listed sources for more information, particularly if you are interested in how light refraction plays into our perception of color in glacial waters!
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1. What color does the water appear to be as you view it? What local environmental processes may be affecting the color of the water?
2. Is the water color more or less intense as you look straight down versus looking out at a distance? Why do you suppose the color looks different?
3.Do you suppose that the color of the water is more intense upstream or downstream from this location? Why?
4. (Optional) Take a photo of yourself on the bridge!
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Sources:
Source 1
Source 2