[Editor’s note: This is an excerpt from Tom Rosenbauer’s great book, Fly Fishing for Trout: The Next Level, from Stackpole Books. It’s a fantastic book, and I really like the graphic style of showing where trout live, which Tom created himself.]
When you get down to the micro level in a trout stream, objects in the water also help you pinpoint where trout feed. The most obvious items are rocks on the streambed, and usually it is not a single rock but a jumble of them, because the current in a river sorts objects by their size and resistance to current. The faster the water, the bigger the object, because smaller objects get pushed downstream in the current until it is slow enough for them to settle out. Thus, slow pools are carpeted with silt, pebbles, and sand, but as you move up to the head of a pool, you find bigger pieces of gravel, then rocks, then boulders. This is not to say you won’t find a big boulder in the middle of a slow pool, but often they don’t stay there because they either get covered up by the finer particles or the lack of resistance on the softer bottom moves them out of the slow water in the next high flow.
Smaller rocks or piles of rocks don’t need much elaboration because you can cover them with a single cast. In other words, a bowling-ball-size rock might hold a nice trout somewhere close to it, but since you can effectively cover the pocket in front of the rock, the slow water behind it, and the mild turbulence even farther back with a single drift, it really doesn’t matter exactly where a trout might be lying. As long as you can recognize the presence of a rock, either by seeing it through the lens of clear water or by the wake made on the surface, it doesn’t matter.
Its when you get to bigger objects, those wider than a refrigerator, that reading the water becomes more helpful. You won’t be able to cover the entire area with a single cast, and because each subsequent cast risks spooking a wary trout, you want to place your first few casts in the best spot and subsequent presentations in less-prime locations.
Most anglers make their first cast right behind a rock, in the cavity region, but in my experience that location would be my last choice, as I have both caught and seen far more trout, and bigger trout, in the stagnation point in front of rocks. At first this appears to be counterintuitive, but when you look at hydraulics and the feeding behavior of trout, it makes more sense.
Trout in front of a rock can see what’s coming, and they also get more of it because food drifting down in the current gets pushed aside by the rock, straining all those drifting goodies away from the cavity region. Yes, the vortices created in the wake rotate current back in behind the rock if the current is strong enough, but that backwash drifts into the trout’s position from behind, so it is not able to see the food and plan an attack to capture it. Of course, if the rock is big enough to create a whirlpool behind it, trout may be found facing downstream (but still with their heads up-current), but the vortices create an unpredictable flow and it’s just not as easy to capture prey there.
Although the stagnation point in front of a rock is seldom long enough to encompass a trout’s entire body, they still seem to be comfortable resting in front of a rock. I believe that because the rear half of a trout’s body is in this lowvelocity area, and also because the force of the water digs a small trench in front of a rock, trout are able to hold their positions here with minimal effort because of their fusiform shape: The wider front half of their body acts like an arrow and cuts the current, and the more flexible rear portion, protected from the current because of the stagnation point, does not have to work as hard.
Almost as productive as the front of a rock are both sides of a rock. Although current picks up velocity along the sides of a rock, most rocks are not perfectly smooth, so any projection from the side of the rock creates a nice cushion of slower water. A trout here has the advantage of seeing more food drifting past than it would lying directly behind the rock. The smoother the sides of the rock and the faster the current, the less likely this will be a good place—but in moderate currents the sides of rocks or rock piles are my second choice for presenting a fly.
We’ve seen that the spot behind a rock can be less productive than other places, because it strains a lot of the food from the current. But what happens as you progress farther downstream from the rock? A lot of it depends upon the size of the rock and the velocity of the current. In fast currents the vortices create standing waves, buffeting a trout out of position. In slower currents the vortices get more predictable and less violent, and the farther you get from the rock, the less dramatic they are because they begin to get diminished by the force of the current outside of the wake.
At some point below an obstruction, the downstream velocity of the water is still reduced but the flow is more stable and oscillations decrease in amplitude. This is another place of great interest to us as stalkers of trout, because the flow is more uniform and trout are protected from the main flow of current, and at this point prey items that flow past the rock are directed back into the wake.
I think this is the reason we catch fish when we cast behind rocks—not because fish are actually lying right behind the rock, but when we cast to that point, our flies pass through the area of the cavity and strong turbulent flow, and then drift into the lower part of the wake where most of the trout actually live. Behind a small object this doesn’t really matter, but because the cavity region is about half the size of the object above it, for a boulder that is 5 feet wide, your entire drift might be in unproductive water. In addition, to get to that spot, you might be casting your fly line over the place where trout are actually feeding, thus spooking the fish before you even get a chance to put a fly over them.
The Multiplication Factor
Objects don’t exist on a stream bottom in isolation, and the macro effects of turbulence—depth, lines of drift, and opposing forces—can either augment a rock’s importance to trout or diminish it. For instance, in a place where water flowing into an object is already strongly turbulent, no matter how tasty a rock looks, the force of the upstream turbulence will combine with the turbulence around the rock, negating any beneficial effects the rock itself might offer to trout. It also makes the hydraulics so convoluted that you won’t be able to figure out what’s happening in the vicinity of the rock. It’s okay to throw your fly there—who knows? But your fly might also end up nowhere near where you predicted it might drift, and only a dozen or so drifts in the area will let you figure out just what the current is doing to your offering.
Now imagine a stretch of water with moderate flow where the main line of drift intersects perfectly with the rock in front of you. Lying right in the middle of a food highway, the stagnation point in front of that rock assumes even greater significance. And imagine how appealing that spot in front of the rock would be to a trout if, in addition to these things, that rock was located in the lower part of a wake from a bigger rock upstream, right at the point where turbulence settles down and becomes uniform. That’s a spot you want to concentrate your efforts. If you are wading a small stream where you can hit every likely place, this is not such a big deal. But if you are floating a big river where you may have to row from one side to the other to hit this spot, it’s to your advantage to anticipate a path that will take you within casting distance of this spot, while the less productive water in other parts of the river slips away as you float downstream.