Brown to Blue: The Eel River’s Dramatic Changes in Hue

Eel River. Photo by Howard King
Eel River. Photo by Howard King

When the South Fork Eel River in Mendocino County turns blue during the winter, it is impossible not to wonder why the river changes color so much over the course of the year.

With first fall rains, autumn leaves falling into the river begin to decompose and release carbon into the water which stains the water dark brown, the same shade as tea. As the rains increase, runoff from the land into the river brings sediment which turns the river cloudy brown. Interestingly though, once the sediment flows out of the river or settles out of the water column, the Eel River turns a brilliant blue. We see this color because the remaining particulates in the river scatter the blue light that reaches the river.  Most other wavelengths of light are longer, but short-wave blue light is often scattered instead of absorbed by the particles it comes into contact with. This phenomenon explains why the sky is blue.  It also is why nonfat milk appears blue!

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About the author

Emily Burns, the League’s former Director of Science, led the research program that includes the Redwoods and Climate Change Initiative. She holds a PhD in Integrative Biology on the impacts of fog on coast redwood forest flora from the University of California, Berkeley.

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2 Responses to “Brown to Blue: The Eel River’s Dramatic Changes in Hue”

  1. Kelly Pomeroy

    I believe it’s the tannin in leaves that stain the water brown.

  2. grant

    Great article! I just drove to the Ave. of Giants and stopped at my favorite spot on the Eel.
    Thank you for the information.


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