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!