[plland]

I would need to see the riffle myself to be sure (some are best left to whitewater rafters and adventurous kayakers...), but I've caught fish right around riffles before. As Green pointed out, the water speed a few feet down is often much slower than it is on the surface due to the bottom of the stream becoming deeper. A trout will hang out in the relatively slower water there, or to the side of the riffle, behind a boulder, and wait there for food to float down the current and into its line of sight. Overall, fast moving water can be hard for a beginner to fish because it's often required to get a nymph deep in such water, quickly, and watching an indicator is often pointless due to all of the different movements on the surface of the water. Most fish I have caught in riffles have been accidental-on the back cast a fish was already taking a bite of the fly.