Color in Mountain Streams

While I was painting this 11x14 inch plein air study of Esopus Creek, I was thinking of a quote by John Ruskin’s (from The Elements of Drawing).

He said that he likes to paint

“mountain streams when the water is shallow, and the tones at the bottom are rich reddish-orange and black, and the water is seen at an angle which exactly divides the visible colors between those of the stones and that of the sky, and the sky is of clear, full, blue. The resulting purple, obtained by the blending of the blue and the orange-red, broken by the play of innumerable gradations in the stones, is indescribably lovely.”

I don’t know if I succeeded in capturing what he was talking about, but he sure has a wonderful way of describing the colors of mountain streams. I’ve noticed that when you look at a stream with gray stones and clear water on a sunny day, there’s far more color beneath the surface of the water than above it.

What I’ve been trying to understand, and am still grappling with (maybe you can help me), is why the stones on a shallow bottom of the stream shift to warm, brownish colors, and then the colors get progressively cooler as you go deeper than about three feet of water. In other words, why does the color of the streambed get warmer than the local color of the stone, and then cooler as it gets deeper?

My hypothesis is that a foot or two of water depth subtracts, through scattering, the cool wavelenths of the light that bounces off the bottom. Therefore, light passing through the water and illuminating the bottom, returns to our eye relatively warmer. It’s also darker because of the light lost to reflection off the surface (which we covered in previous posts: part 1, part 2, and part 3.).

I hesitate to show a Sargent in the same post as my own work, but he captures and accentuates these effects so beautifully. Note the warm stream bottom at the right of the painting below, called "Dolce Far Niente." The cooler colors at left are not from depth effects so much as from the addition of cool sky light off the surface.

Why are the deeper parts cooler, even in the absence of reflection? The second half of my theory is that in deeper water we’re seeing less of the light simply bouncing off the bottom; we’re seeing more of the blue light that has been scattered in all direction—the same reason that infinite depths of air looks bluer.

I don’t know: maybe I’m wrong, or maybe someone can explain it better. Anyway, I love the Sargent above, which shows a stream that's not deep enough for the blue-depth color, but it has the warming effect in the shallows, and a miraculous feeling of the image of the bottom distorted by the rippled surface of the water.
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The Sargents are from the Brooklyn Museum, link.
Previous GurneyJourney posts on transparency of water, link.
and water reflections, part 1, part 2, and part 3.