Photo Credit: Ryan Mollnow, USFWS Mountain-Prairie 2004. Derivative: cropped by dkp. License CC-by 2.0.
It took quite a few more searches (and several articles perused) for more clues, that finally provided me something of an answer. With the last set of terms, the image search result gave up a set of pictures showing full-blown cottonwoods going to seed. Finally, the fuzzy, white "summer snow" I sought:
*cottonwoods
*cottonwood trees
*towhead
*river towhead
*towhead with cottonwoods
*cottonwood trees in seasons
*cottonwood trees in all stages
*cottonwood trees white
*cottonwood trees full blown seed
*cottonwood poplar thicket summer fuzz white
*ohio summer snow cottonwood seed
I haven't tried Google, but I know that Facebook would have categorized my thoughts immediately. One mention of the word "towhead," and I would see memes of blond kids, ads pushing Mark Twain books, pictures of islands, documentaries about riverboats, and travel ads for Missouri and Mississippi River destinations. Facebook reads my mind.