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Recently we visited the alps in Bavaria. We hiked near a lake which had a lot of foam swimming on it. It was not spray from the water but rather a really dense dirty foam. Comparable to shaving foam I guess.

It looked similar to this (this image is from the internet and was not taken by me while being at the lake):

enter image description here

Where does such foam originate from?

P.S.: I've seen such foam also on other waters already. It just was somehow this time that it made me wonder.

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    I'm voting to close this question as off-topic because it is not about outdoors, but about a phenomenon observed outdoors. Not everything that can be observed outdoors is on-topic on The Great Outdoors or the number of potential questions to ask would become totally unbounded. – gerrit Apr 27 '17 at 13:42
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    As for my close-vote, see this meta question. – gerrit Apr 27 '17 at 13:51
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    I think it's a fair question as it's it's a first step to the question "could this be harmful" - which, in my experience, is normally asked when people encounter it in water they've just been in. – Niall Apr 27 '17 at 16:06
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    @gerrit given that your proposal is unresolved on meta (at best, really it looked closer to rejected), it doesn't make sense to use it as an argument for closing. – Niall Apr 27 '17 at 16:11
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    @Niall My link to the meta question predates the answers or votes on the meta question. It's not so much an argument as to a link to a more extended description of why I voted to close. I can recognise many people disagree with me. – gerrit Apr 27 '17 at 16:23
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It's "foam line" (though I believe there are other names)

It occurs when there are surfactants in water, which is then agitated, resulting in foam forming. This foam then tends to clump together and collect debris along the way.

It happens naturally, especially in fast-flowing rivers or larger lakes in boggy areas.

It can also be caused by pollution - eg. from agricultural run-off, human waste or industrial pollution.

(Note: it isn't an algal bloom as mentioned in another answer, but can occur in tandem as 1. Organic matter in the water can cause both foam and algal bloom 2. As the algi decompose it adds organic matter which causes foaming.)

Link to Wikipedia article

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    Huh, I never realized this could be naturally occurring. I thought it was always the result of phosphates and surfactants in pollution. – Malvolio Apr 27 '17 at 16:49
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    You'll see this in natural areas that presumably are not very polluted by humans @Malvolio. Also as the Wikipedia article notes, a clue as to whether these foam patches are from human pollution or not is whether or not the foam is concentrated only in one area, or if you see foam patches regularly through the area you're in. This is essentially the difference between 'point-source' vs. 'non-point source' pollution, with human sources of foam patches being, generally, point source, and naturally sources of foam patches being NPS. en.wikipedia.org/wiki/Nonpoint_source_pollution – cr0 Apr 27 '17 at 17:11
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    @cr0 I think for foam is kind of a weak rule - it's normal for naturally occurring foam to get blown by the wind into one corner of a lake or for it to happen in eddies where a river flows into a lake - or in any eddy on a river. I.e. it's very dependant on conditions and geography. – Niall Apr 27 '17 at 17:23
  • Do you mean "human waste" as in feces (then why can't it be animal waste) or other forms of waste? – Azor Ahai Apr 28 '17 at 7:02
  • @Azor-Ahai it can. A significant amount of human waste will only come from sewers that dump untreated waste. A significant amount of animal waste will come from manure spread in fields - which is (part of)what I meant by "agricultural run-off". – Niall Apr 28 '17 at 13:32
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It's similar to sea foam; nothing to worry about. This usually results from the build up of broken down organic matter that has dissolved in the water and foamed up due to turbulence (i.e. wind/waves, tides, ect.). For example, breakdown of organic material including microscopic critters, like algae, yields proteins that can foam up when agitated - kind of like making meringue from egg whites. This foam can collect along the shoreline and accumulates specks of dirt and debris.

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I used to have a marine (reef) aquarium. One part of filtration is called a foam fractionator. By blowing fine bubbles in a thin column, you produce a great deal of surface area where air and water touch at an interface. This affects the entire volume of water in the cup so stuff in the water can’t circulate away or avoid the (normal) surface. Thus the effects below are greatly enhanced.

Some molecules — including many proteins, especially cell membranes from leftover food and waste — have spans that are hydrophobic and spans that are nydrophilic on the same molecule. That's bound to happen on any long complex protein.

These will gather at the air/water boundary, which it loves better than being totally under water.

A collection of this stuff on a bubble makes the bubble wall stay, in exactly the same way a soap bubble forms. Soap is in fact a molecule of this type.

Once such boundaries start collecting, lots of other stuff will get swept up in it and stick, as well. In your picture you see sticks and leaves. The same idea occurs on a microscopic scale with tint bits of whatever is floating in the water.

In the aquarium, the cup is raised or lowered until the “head” of foam can spill out, removing it from the water.

The same foam fractionation can certainly occur naturally. A piece that floats around will “snowball” slowly as there is a tiny air/water interface surrounding it. Where there's a turbulent fall of water, it more closly resembles the machine version, forming bubbles which are left to recirculate in a small volume.

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