High Sierra Topix

A see a lot of that foam, particularly in the spring, when water is running off the hills into the streams.
This is a reasonable description:
https://www.rappflow.org/resources/faq.html
Kinda the same thing as seafoam:
https://en.wikipedia.org/wiki/Sea_foam

From RappFLOW FAQ wrote:Unnatural foam from human activity usually:

• appears white in color;
• gives off a fragrant, perfumed or soapy odor; and
• usually occurs over small area, localized near source of discharge.

The foam in the photo is white and very localized. Perhaps there was soap residue in the vegetation or soil along the shore. It's hard to say from just a photo.