The Color of Water
The next time you are standing by a lake, pond or the ocean, ask yourself why it looks the way it does. What makes oceans and lakes look blue or green and sometimes black? Why do rivers most often appear brown? When you cup some of the water in your hand, it's colorless, but look at the body of water and it's a color. The most obvious answer is that it reflects the sky since you can see reflections of clouds also. But reflection is only part of the answer.
It is true that part of the light you see is a surface reflection of sky light. The amount of this sort of reflection, though, depends on a number of things such as your viewing angle. On the other hand, much of the color you see comes from the water's depths.
White sunlight contains all colors.. Water
absorbs some of these colors more than others. But you need a lot of water
to notice this effect - which is why water cupped in your hand has no color
at all. In a large body of water though, the water molecules quickly
absorb red light, but they don't absorb blue light nearly
as well. So in this case, more blue light is transmitted through the water. It's actually the same principle behind why the sky is blue.
The blue effect is often increased because ocean and lake water usually contains floating particles of dirt, and dead and living plants and animals. They help reflect the blue light back upward - so that the lake or ocean looks blue. The blue-green of tropical waters is due entirely to absorption of red light rather then suspension of particles. It's a different blue than the blue of the oceans off the northeast coast, whose blue is caused by suspended particles and plants.
So now we know why some bodies of water appear blue, but what about the other water colors, like gray, brown and black? Different colors can be produced by specific things suspended in the water. Mud acts in the opposite way from water -- it reflects red light more than blue. So muddy water looks brown. Since rivers are big transporters of mud, they usually appear brown. Peat dissolved in water absorbs almost all the light that touches it - so a lake full of peat may look black. The ocean often usually looks gray on a stormy day. Partly, that's reflected gray sky - but it's also because clouds filter out a lot of the sun's red light before it ever reaches the water.
Colors of water can be pretty distinctive. The Black Sea is so named because of its color, a very dark almost black color. In this case, it's the almost constant fog over the Black Sea that absorbs incoming light and makes the sea appear black. It was enough to make the Turks and many other sailors want to avoid what they saw as an evil looking sea!
Copyright © 1999 Kathy Miles and Charles F. Peters II