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What causes the surface of the lake to appear darker in some places?

Firstly, I know that it's not fishes that cause it. Secondly, the dark surfaces move from time to time and thus are not associated with the depth of the water or the structure of the lake bottom. Lastly, I may say they are due to a kind of ripple distortion. What causes this distortion and what is its mechanism?

Please, provide more explanation than "the wind", for example.

In this photo, look at the far area; it's darker somehow.

enter image description here

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  • $\begingroup$ I often wished they were fish, but in my experience they were generally reflections or shadows of clouds... $\endgroup$
    – tom
    Commented Apr 16, 2015 at 21:19
  • $\begingroup$ The question becomes: why does the water change texture? I posted that as a related question: physics.stackexchange.com/q/348550 $\endgroup$
    – Schroeder
    Commented Jul 25, 2017 at 19:17

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It is an interplay between the wind and the shoreline, and basic laws of reflection.

As you can see in your photo, where the water surface is still, you see a reflected image of the skyline - lighter for the sky, darker for the buildings.

Where the water surface ripples, you get reflections "from everywhere" - some from the sky, some from buildings, etc. that average is darker than the bright sky.

Another factor relating to this: when you look at a rippled surface, the average angle that you see is lower than if you are looking at a smooth surface. Imagine a smooth sine wave ripple: when you look from the side, you see more of the surface "facing you" and less of the surface "facing away". Now Fresnel's equations tell us about the coefficient of reflectivity as a function of angle - and if we ignore polarization, the reflection goes up at the angle of incidence gets more oblique.

On the rippled surface the average angle of incidence is less oblique and you see less reflection from the sky. This is why these patches look darker:

enter image description here

For larger angles $\theta$ the reflection coefficient is larger - but when you look at the ripples, you can see that you "favor" the water that is facing you more directly - this means you see less reflection, and you are looking more "through the surface at the deep below" than "at a reflection of the sky".

Incidentally, the Fresnel equations can be found on this Wikipedia page, where you also find this diagram. Here, $R_p$ and $R_s$ are reflection coefficients for different polarization angles. Although polarization will change in the sky throughout the day (single Rayleigh scattering leaves the light polarized), on average you can ignore its effect for the purpose of understanding this explanation:

enter image description here

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    $\begingroup$ Good answer.But what causes these short length ripples in the first place ?.That is my intended question.To be clear on the depth matter;sometimes one can see a patch ripples that make up a darker area ,as you've explained,but with soft light (i.e. normal) in the middle of it. I'll try to supply more photos. $\endgroup$
    – Fadi
    Commented Apr 18, 2015 at 15:57
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    $\begingroup$ The ripples are caused by the wind. Local variations in wind are very common - ask any sailor. In fact, when you are sailing in light winds, this phenomenon is how you can see where "the good wind is", and use it to gain an advantage (of course some people use LIDAR for that... I'm old fashioned). $\endgroup$
    – Floris
    Commented Apr 18, 2015 at 16:06
  • $\begingroup$ but these variations are so local that these darker areas i.e, the parts of short length ripples ,stretch sometimes across few square meters!.Is that possible ?. $\endgroup$
    – Fadi
    Commented Apr 21, 2015 at 19:14
  • $\begingroup$ Yes, that's absolutely what the wind does. See for example this earlier question and the answers / links therein. $\endgroup$
    – Floris
    Commented Apr 21, 2015 at 19:42
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It depends.

It could be wind, simply the lighter section is rougher and the waves scatter light back to you while the flatter section appears darker because the light is scattered in a different direction.

It can also happen where waters mix. A fresh water stream merges into an ocean, a flowing river meets a shallow stagnant area or (as below) river water with suspended sediment, which scatters light, meets deep ocean water.

enter image description here

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    $\begingroup$ It could also just be sudden changes in depth -- light water may have a sandbar under it or something. Or it could be due to difference in what is under water -- plants or dark rocks or something. $\endgroup$
    – tpg2114
    Commented Apr 16, 2015 at 19:32
  • $\begingroup$ In this image I suspect the lighting difference is totally from the surface, it is at such a low angle of incidence and the water is probably heavily sedimented (guessing from the location) $\endgroup$
    – Martin Beckett
    Commented Apr 16, 2015 at 19:33
  • $\begingroup$ I would agree with that from this image. I was just adding more possibilities that might happen in general. $\endgroup$
    – tpg2114
    Commented Apr 16, 2015 at 19:41
  • $\begingroup$ the case I've talked about has nothing to do with depth because they keep shifting within few hours our minutes sometimes. $\endgroup$
    – Fadi
    Commented Apr 16, 2015 at 20:36
  • $\begingroup$ Since it moves around, it cannot depend solely on depth. That doesn't mean that it has nothing to do with depth. Different wind conditions might cause different sets of depth-dependent (and flow dependent) ripples. $\endgroup$
    – BowlOfRed
    Commented Apr 16, 2015 at 20:42
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Obviously darkness of lake water depends on depth of lake, impurity in water and many other things.

What causes the surface of lake to appear darker in some places?

It depends on two things:

  1. position of observer;

  2. position of sun in the sky.

If sun is nearer to horizon then the amount of light, reflected from under water, reaches to observer is less for Observer 1 standing on the Sun's side compare to Observer 2 standing opposite to the Sun.

So, lake water looks darker to Observer 1 than Observer 2.

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