Here's a link to Brad Befus' Cottonwood Seed Fly with CDC.
[url=http://www.nkyflyfishers.org/Docs/CDC%20Cottonwood%20Seed.pdf:6fa07]http://www.nkyflyfishers.org/Docs/CDC%20Cottonwood%20Seed.pdf[/url:6fa07]

If cottonwood seeds are on the water, this is deadly.
Joe