Detail of the surface of an accumulation of foam on the canalized river <a href="https://en.wikipedia.org/wiki/De%C3%BBle" class="extiw" title="w:Deûle">Deûle</a> downstream of Lille in the North of France (here just downstream of the lock of the Grand-carré, at the exit of Lille, near Lille/Saint-André). This foam means organic and/or chemical pollution; it can evoke the presence of a detergent or similar product in the water (quaternary ammonia, etc.).
The phenomenon still occurs episodically, sometimes accompanied by a particular smell. <br>In the 1960s, it was much more frequent and intense, and the layers of foam could exceed 1 or even 2 m in thickness. At that time, there was no sewage treatment plant in this area and the layer of foam was much greater.
<a href="//commons.wikimedia.org/wiki/User:Lamiot" title="User:Lamiot">Lamiot</a>
CC BY-SA 4.0
<span class="int-own-work" lang="en">Own work</span>