Matching the Hatch in Fast Water

The majority of my time spent fishing is on the St. Joe River in North Idaho and its tributaries.  Over the years I have discovered that my most productive water is usually the riffles and chutes leading into a deeper hole.  When the cutthroats are in a feeding mood, they seem to move the the front of the hole and even further up into the riffles and diamond water above.  I catch a lot of fish in the faster moving water.  When the fish are there, and an insect is drifting naturally at or near the surface, the fish do not have much time to decide to strike or not.  I use this to my advantage. Because their decision making time is so short, I believe the most important aspect of trying to match the insects in the water with an artificial fly, is to get the size right.  Many times this is enough to motivate a fish to hit your fly.  The second most important aspect is the color, especially the tone, light to dark.  I believe tans and grays are similar.  Black, brown, and darker colors also appear to work the same.  So try and match the lightness or darkness of the naturals.

The actual silhouette of the fly is of little importance in the faster water.  By this I mean a trout in these conditions will hit a darker colored mayfly imitation when darker colored natural caddis flies of the same size are in the current.  On the St. Joe in the last few years the 'Purple Haze' is being used more and more effectively.  I have never seen a natural insect that this pattern is trying to imitate. However, in swift water conditions, I believe that fish will mistake this for any darker colored natural insect of similar size if they are hatching.

Several weeks ago my friend and I were fishing one of my favorite holes and I moved up into the riffles above when I saw fish rising.  I was able to capture a larger than normal chironomid that was about the equivalent size of a #16 dry fly.  All of my chironomid patterns I had with me were #18 and smaller.  So I tied on a #16 BWO pattern and began catching fish.  When my friend asked what fly I was using, he was surprised when I told him.  He commented "a Blue Winged Olive should not be working, they are not hatching."  I explained to him that I knew that, but since we where in the faster moving water of the riffles, the size and color of the fly was enough to trigger a strike when the fish only have a split second to decide.

I would be curious to know if any other fly fishermen have seen the same thing when fishing faster water.  Comment below and let me know your opinion.