Neither of the above!
This post explains a perfectly natural, Spring 2020 occurrence in the Waychinicup River
It also offers some “truth” about the alleged health benefits of drinking Guinness.
One of our favourite parts of Waychinicup National Park is just a few hundred metres upstream of where the river flows into the magnificent inlet that is Waychinicup’s signature feature.
Surprisingly few visitors make the very short, albeit just slightly “tricky” walk (you need to be sure-footed and two-armed to do it safely, unassisted) to where the river encounters massive granite boulders/outcrops.
Here, deep pools form, and water is still present even during dry times when the modest little river is not flowing.
When the Waychinicup River roars back into action, usually in Winter and Spring, it carries a deal of dissolved and solid-but-partially/potentially-soluble organic matter.
Australian “bush” plants are typically very rich in tannins – so much so that their tannins have been commercially harvested/farmed.
Dissolved in watercourses, those tannins darken many Australian waterways, most especially “seasonal” or “intermittent” creeks and wetlands.
DOC – dissolved organ carbon – from the breakdown of those tannins and other organic matter in bark, twigs, leaves etc can produce surfactants: natural detergents.
They form foam, especially when flowing waters meet rocks, bridges and pooled, deeper water where organic matter has settled, accumulated.
Click here for a good explanation, from a U.S. source (this phenomenon is not uniquely Australian) and this for an Australian overview.
Occasional abundances of foam and/or very dark water are not necessarily symptoms of pollution/environmental degradation.
When the water’s edge looks healthy, no dead fish are floating by, and nothing smells “bad”, those “suds” and that “stain” may be symptomatic of ecological good health, even a contributor to it.
Guinness footnote
Advertisements for Ireland’s emblematic “dark stuff” used to declare “Guinness is good fo you”.
Such ads no longer run, but the belief persists.
The “truth” is contested, still.
That said, “Push Doctor” speaks good sense, I think:
it’s important to remember that this is not the 1920s and Guinness has never been a health drink.
The probably-good doctor’s full assessment of Guinness is here.