In theory, fly-fishing is a simple sport: Pick a body of water, choose a fly-fishing rod, select your “fly” (or bait), tie a secure knot, cast your line and, hopefully, land a fish on the other end.

I’m not suggesting you drift a pair of dry flies through fast water or stained water. The double dry rig works best when fishing slow, clear water that offers the potential for rising fish – if you ...