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I recently visited Slovenia where I saw several rivers with a strong blue-green color. Here is an example from one of the tributaries of the Soča river near Kobarid. The locals refer to it as an "emerald" color.

Emerald blue-green river in Soča valley

Notice how the color gets more intense at the deeper waters. What causes this color? A web search shows that the two most cited reasons are blue-green algae and "limestone particles". I'm guessing that the more plausible explanation in this case is the limestone one, because this area is dominated by limestone. But, is it really "particles"? I would think that the calcium carbonate would dissolve in the water, instead of being suspended as solids.

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According to the website Putting the Emerald in Emerald Lake – Where do the colours come from?, the reason for the colour is the same that occurs in lakes, it is due to what is known as or similar material to 'rock flour' - consisting of

of fine-grained, silt-sized particles of rock, generated by mechanical grinding of bedrock

and relevant to a river environment

When the sediments enter a river, they turn the river's colour grey, light brown, iridescent blue-green, or milky white.

The colour is due to:

the suspended material distorts the wavelengths of light, reflecting back more of the green and blue end of the spectrum

These usually originate from glacial melt, even though there are no glaciers in the Julian Alps presently, but there used to be, resulting in according to the WP6 Hydro-geological upgrade and improvement of the GIS of the sources of drinking water webpage as having

The intergranular aquifer in the alluvium of the Soča/Isonzo River, which is composed of fluvial and fluvioglacial gravel and sand sediments with individual lenses of conglomerates

further explained in the article Bovec basin (Upper Soča valley, Slovenia), as having

Quaternary sediments are represented from bottom to top by partly lithified glaciofluvial sediments, overlain by lacustrine chalk. During the Holocene, the chalk was partly eroded (in some areas totally) and covered by glaciofluvial sand and gravel which are weakly cemented in some parts into conglomerate and by unconsolidated moraine (till).

Potentially, plenty of source material. The particles are small enough to be suspended in the water of the Soča River.

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  • $\begingroup$ This explanation explains why river water has this color. However, I noticed that many springs are green right from the start, and they did not have sufficient time to accumulate the fine sediments. Could this be the only explanation? $\endgroup$
    – Gimelist
    Sep 25, 2014 at 6:12
  • $\begingroup$ If the springs originate in the same rocks then it is possible. Having said that, one thing I have always found was that there usually were often other contributing factors to any observed phenomena. $\endgroup$
    – user889
    Sep 25, 2014 at 7:20
  • $\begingroup$ Also look here: world-of-waterfalls.com/american-southwest-havasu-falls.html where the water have similar color, but this is definitely not in a glacial environment so I doubt the contribution of rock flour. They note that the color is due to calcium carbonate (travertine in their case). I tried finding absorption spectra of dissolved calcium carbonate but no success. $\endgroup$
    – Gimelist
    Sep 25, 2014 at 7:31
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    $\begingroup$ Yes it can be chemical input - volcanic lakes can often be a bright green colour due to acids, however I can confirm the glacial flour as one cause. You often see glacial flour colouring in Alaska. Often the water appears a bit milky in addition to green (I assume you need a certain amount of suspension to get the milkiness). $\endgroup$
    – winwaed
    Sep 25, 2014 at 13:06
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    $\begingroup$ The amount of limestone or calcium carbonate in the water is based on equilibriums between the acidity, temperature, dissolved calcium carbonate. If CO2 is off gassing in the river some calcium carbonate would start to precipitate out of solution. $\endgroup$
    – user824
    Oct 21, 2016 at 0:06
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The turquoise to sky-blue color appearance of freshwater bodies are nicely explained with examples in a 2016 paper by Kumar entitled "Study on turquoise and bright sky-blue appearing freshwater bodies".

In this paper, the author explains the cause of these shades of greenish-blue shades found in freshwater bodies around the world and proposes a classification/categorization of such freshwater bodies. the author has classified different turquoise to sky-blue appearing freshwater bodies into two major classes:

1)Natural

2)Anthropogenic

Such water bodies with causal factor of natural origin are placed under 'natural' and causal factor of non-natural (anthropogenic) origin are placed under 'anthropogenic' category. The 'natural' category has been further sub-classified into three sub-class : a. Glacial Fed (streams, lakes appears turquoise to sky-blue due to scattering of light by glacial flour (rock flour), ex- Phoksundo lake (Nepal), Moraine lake (canada), etc . b. Volcanic and Hydrothermal: Lakes and rivers in volcanic region and of hydrothermal origin also appear turquoise to sky-blue. For example, kawah Ijen (Indonesia), Yudamari lake (Japan) which appears blue as a result of scattering from sulphate and ferrous particles. Rio celeste in another river in Costa Rica which appears turquoise as a result of scattering from aluminosilicate particles. c. Calcium carbonate rich: Calcium carbonate rich freshwater bodies also appear turquoise to sky-blue as a result of scattering from suspended calcite particles. Example,Havasu creek (grand canyon), Bear lake(Utah), Plitvice lakes (Croatia).

  1. Anthropogenic : The author has discussed two water bodies appearing turquoise to sky-blue. Lukha river (Wah Lukha) which flows in meghalaya (India) turns turquoise every year in winter season, real cause not known but coal and limestone mining cited as a reason. Wilge river (S Africa) flows in coal mine affected area appears turquoise and aluminium has been cited as causing agent.

    I would recommend reading the paper if you are interested in getting a deeper understanding. Picture 1. Different Glacial-fed freshwater bodies. 2. Calcium carbinate rich freshwater bodies. 3. River appearing turquoise as result of anthropogenic activities. different glacial lakes Calcium carbonate rich anthropogenic

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    $\begingroup$ Can you summarize the findings of the paper in your answer? Otherwise your answer is 'link-only' which is discouraged here. $\endgroup$
    – kingledion
    Jul 13, 2017 at 20:04
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Dissolved micro particles of calcium carbonate, aluminum etc in the water coupled light reflection and resultant spectral wave cause various color in the lake water. It is unlike algie (red) found in Red Sea. Light reflection and depth is a major factor because the same water look regular water in s test tube.

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