By Elise Matera, UC Davis Tahoe Environmental Research Center
Lake Tahoe is blue because it is deep and contains little algae. While Tahoe is also famously clear, UC Davis Tahoe Environmental Research Center (TERC) scientist Shohei Watanabe discovered in 2015 that this is not the primary cause of its blueness. The lack of sediments keeps Tahoe clear, but the lack of algae keeps Tahoe blue. Our perception of blue color also has to do with the physical properties of light.
Out of all the colors on the spectrum, blue light is just the right wavelength to swerve around water molecules and sediments and penetrate deep into the water.
White light from the sun is made up of all the different colors of the rainbow, from red to green to violet. Each color of light has a different wavelength. Red is the longest and violet is the shortest. When white sunlight hits the surface of Lake Tahoe, it separates into all its different wavelengths in a process called refraction. Refraction is also what creates a rainbow in the sky.
The longer the wavelength of light, the more quickly it is absorbed into the water as heat. This means red, orange, yellow and green lights are absorbed into the water producing just enough heat for a quick dip into the water in the summertime.
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Out of all the colors on the spectrum, blue light is just the right wavelength to swerve around water molecules and sediments and penetrate deep into the water. Once the blue light wave reaches the very depths, it is reflected to our eyes, which tells our brains: “Wow, that’s blue!” Shallower lakes allow longer wavelengths like green to reflect to the eyes, which is why the edges of Lake Tahoe appear more turquoise.
Watch a video about why Lake Tahoe is blue
Courtesy UC Davis
But Tahoe’s depth alone is not the only reason it is so blue. Its clear waters contain little algae and few sediments, which in high quantities could make a lake look green or brown. Algae, like many other plants, uses chlorophyll to photosynthesize. Chlorophyll absorbs red and blue light, and not green light, which is why green is reflected to our eyes. If Lake Tahoe contained lots of algae, it would be much greener.
While sediment is not the primary contributor to Lake Tahoe’s blueness, the type, color and amount of sediments in a lake can change its color. The murky waters of the Mississippi are almost opaque in some places because of the amount of sediments it carries, reflecting more wavelengths to our eyes.
It is important to keep Tahoe blue and clear and knowing the causes of both will help us better protect it. | tahoe.ucdavis.edu
