|Photos by Marty Horowitz 1/13/2016|
I've posted about this before (here, with Deep Purple); (here, with Marty-art). I'll reproduce the Anne-approved explanation here: (entire post, with photos, here)
Water normally sticks to itself. This allows tiny critters like Water Striders to 'walk' on water. As tiny particles - dissolved organic carbon, to be exact - get washed into the creek after it rains and get further churned up as they flow downstream, they reduce the stickiness of water to itself and allow air to be trapped as bubbles. The bubbles then congregate on the surface where the flow isn't as swift. You can sometimes see inches of thick foam around rocks and near the 'shore.'See you in the canyon!
The source for this organic carbon is - you guessed it - anything that was once living. Algae, plants, animals - we're all carbon-based life forms. Bottom line: harmless and 'natural' in Sabino Creek.
Yes, detergents, soaps, fertilizers, and other substances can cause foam; and that's not usually optimal. I'm reasonably certain that good ol' dissolved organic carbon is the source for the soapy foam on Sabino Creek.
(You might wonder what 'inorganic carbon' is. Best example: diamonds. Made of carbon, but not alive. If you find any when you're panning, be sure to let me know.)