For Christmas, this year, my wife got me "The Complete Book of Western Hatches". With a picture printed off the new printer (came at the same time the computer came) in hand, I was determined to find out what it was.
The best match I could come up with is (and you're going to love this from Mr. Entomology Stupid):
Common Name: Blue-winged Olive
Mel (Ranbow Chaser) was right with the Mayfly response. Without putting it under a microscope the best I can come up with, from the pictures in the book, is a Dun.
So, scary as this might seem, maybe I can shave a little of the edges off the"Mr Entomology Stupid" guy and actually understand some of what you dedicated match the hatch guys are talking about.
Now, what a Blue Winged Olive is doing around the American River is still a mystery to me. Maybe the Steelhead eat them? They are just a Rainbow Trout, after all. Another mystery to be solved.