Years ago, I was sitting on a log in an old mill pond in the Trinity Alps, northern California.  I was just looking at the creatures around, when some type of flying insect came around, one I had never seen before.  It was about an inch or so long, had a slender body, translucent wings, rather non-descript color--tan-ish--and made minimal or no sound when flying.  The remarkable thing was its flight.  I got a close look at it from about 2-3 feet away, as it hovered right there, and it appeared to have wings attached to the center-top of its body, which were rotating like helicopter blades.  I couldn't believe this was possible, so kept staring closely at it, but that was what I saw.  Then the insect took off. 

A few minutes later, it came back and hovered again, and I got a second look at it from close up.  There was no mistaking it--the wings were on top, they appeared to be rotating, but moved so fast I could not tell if there were 2 wings or 4, or exactly how the motion was happening.

Later, I read an article in a magazine, which I think was National Geographici about just such an insect.  Now I can't remember its name.

I do remember thinking, "All these years I thought I was crazy or something, but here it is!"  I can't remember the details of how its flight was accomplished.

Can you please tell me what this insect is?

The only thing that springs to mind is the Helicopter Damselfly:
http://en.wikipedia.org/wiki/Pseudostigmatidae
http://www.arkive.org/helicopter-damsel … eo-00.html
It's wings don't actually rotate like a helicopter (a 'wheel' in nature is extremely rare), but they certainly look like it.
What is unusual is spotting one in norther california, as their range is reported as extending as far as Northern Mexico.