Yes those are champagne corks. My wife and I had two cases left over from our wedding last year. Every now and then we just pull one out and drink a bottle. We are down to about 5 bottles. I have kept every cork to use for tying. I used a few already that I cut way down on some bass poppers, but I thought it would be cool to leave them whole and make a really big popper. So, those flies are the result.
Now, if the rivers would drop...