Why is it when I drive by the river in the winter, it appears a dreery dark grey. Even in the sunlight.

But in the summer, it appears blue?

Because, Sherasi, water reflects the color of the sky. In winter it's almost always grey, and in summer it's usually blue.
But, then again, maybe not.

Well, JV, in winter, the sky could be a bright blue with sun and the water is still dark grey.

Maybe it has something to do with the cold. Did you notice how the air is so much clearer and sharper in the winter with the crisp weather, and so much heavier in the summer? Maybe the cold water reflects differently, and brings up the colors of the bottom of the lake or river better.
Maybe it's also because of the grime and sludge that comes from snow melting into it, the salt and pollution and such.
But you know, I just don't know.

What a perfect question for the science forum

My suspicion is that it has to do with the angle of the sun relative to the earth & water.
(In fact, I guarantee it because that is the only reason winter is different from summer... but I think it is most likely a direct effect, not an effect due to the cold.)
Part of the reason I say this is that water also seems the bluest near the middle of the day in the summer, and the grayest in the morning and the evening, agreed?
I've got some inklings as to why this would be, but I'm going to mull them over a bit before I post them (hope you don't mind suspense).

Ok, so I've mulled it over and have a couple ideas, but first I'll give a little background (bear with me through whatever parts you know).

Light comes in a variety of wavelengths. The shortest wavelengths we can see are violet, the longest we can see are red. In between, the colors are in the order of the rainbow.

Water is blue because it absorbs red light (leaving the blue to pass through it to your eye or be scattered to your eye).

As light passes through air, it is scattered. Shorter wavelengths are scattered much much more than longer wavelengths. For blue light, it is scattered so many times that it seems to be coming from all over the sky, instead of its original source (the sun). Picture billions of tiny mirrors reflecting the light all over the sky towards other mirrors until one of those mirrors reflects it to your eye. The longer wavelengths (redder colors) aren't scattered nearly as much. They come pretty much straight to you from the sun.

The situation above is midday during the summer, when the sun is shining straight down and doesn't have a whole lot of air (comparatively) to go through before getting to you. At sunrise and sunset, the light has to pass through a lot more air to get to you. In this case, the blue light is scattered so much that a lot of it neer makes it to you. With this much air, the scattering of the redder colors becomes significant enough that the sky looks reddish.

In the summer, the angle between the earth's surface and the sun are close to perpendicular. In the winter, the light comes in at an angle closer to the earth's surface (like it does is early in the day and late in the day). This means that there will actually be less blue light reaching you (like there is early in the day and late in the day) than there would be in the summer. This is why the sky isn't quite as deep of a blue in the winter, even on clear days, as it is on a clear summer day.

Now this is where I start to speculate.

Because there is less blue light available, there is less blue light to be reflected back through the water to you, so the water appears less blue (this is sort of like looking at a blue object with a red-tinted flashlight... it looks black because there is no blue to reflect).

Something else that may play a role (although I suspect less of one than the reasoning I explained above):
Again, the Sun's light comes in at an angle closer to the ground during the winter. Because of this, it is probably scattered out of the water without having travelled through as much water. Since it didn't travel through as much water, not as much red light was absorbed, and the water looks less blue (to prove to yourself that more water to pass through = more absorption/more blue, compare the colors of water in a glass (colorless as far as you can see), water in a bathtub (very pale blue), water in a pool (bluer), and water in the ocean (blue).

I hope that makes sense. Please ask for clarifications on anything that doesn't.

Anyone see any problems with my speculation or have a better idea?

I was also thinking about possible reasons.

I know that when water gets cold, it gets denser (hence the eventual ice it becomes). So maybe closer molecultes refracts light at a different color light wavelength.

Oh, and just a not to Johnny... water is not blue because of reflecting the sky. It is blue because it absorbs red light. It just absorbs so weakly that you need a lot of it (more than a glass) to notice. For another example of this, look at a piece of glass. looking through it like a window, it is clear and colorless. looking through it edgewise (the long way) it is usually green (depending on what impurities are in the glass).

Sher - Large amounts of (pure) ice are also a very deep blue, so I doubt that this is the reason. And just a note - ice is actually *less* dense then water (which is why it floats).

Oh of course it is less dense! <smacks her forehead> I knew that.. duhh....