Book Excerpt: “The Orvis Guide to Finding Trout”
In fluid mechanics, flow in a liquid is described as either laminar or turbulent (or some consider a transitional state, which is a bit of both). In laminar flow, particles move in smooth paths in layers, with no mixing of adjacent layers. Everything moves independently, like a deck of cards thrown on a table. The velocity, pressure, and other flow properties remain constant. In turbulent flow, the layers mix and cells of fluid move off in other directions, interacting with each other in a chaotic manner that is nearly impossible to predict.
Although I have seen instances where angling writers write about laminar flow in trout streams, stream hydrologists say it doesn’t exist in natural streams on the scale relative to the size of even a trout fry, because the roughness of the bank and bottom, and obstructions in the water, constantly mix the water. In water flowing over a perfectly smooth rock, you can find an almost microscopic layer of laminar flow, but that’s as much as you’ll find in a natural stream. So even though the middle of a slow, smooth-flowing river appears to be laminar, there is always some turbulence present. It’s just a matter of degree. And we can’t always predict what will happen. In fact, physicists don’t even understand turbulence very well. In a famous but perhaps apocryphal story, Werner Heisenberg, the winner of the 1932 Nobel Prize in physics, best known for his famous uncertainty principle, was asked if he could ask God two questions, what they might be. His answer was, “Why quantum mechanics? And why turbulence? And I’m pretty sure God would be able to answer the first question.”
We know that turbulence happens when water slides by a solid object in a parallel direction and interacts with the object first at a molecular level, where water molecules stick to the object. It’s called shear stress. Water is a viscous liquid, so this layer interacts with the layers above it, forming globs of turbulence that mix with other layers above it in an intricate ballet. This layer of disturbed flow is called the boundary layer and its exact dimensions are impossible to predict. But at any given point, its upper limit is where the main downstream flow of a river is no longer influenced by mixing and turbulence below. According to ecologist Steven Vogel in his book Life Moving in Fluids, “most biologists seem to have heard of the boundary layer, but they have a fuzzy notion that it is a discrete layer rather than the discrete notion that it’s a fuzzy region.”
So if all flow in a trout stream is turbulent, and even physicists don’t understand how it works, how the heck are we supposed to predict where a trout will be? Luckily, turbulence comes in many flavors, from slight to extreme, and we can predict what will happen on a macro level when moving water interacts with solid objects. And even though turbulent flow is chaotic, it tends to repeat over and over in the same spot with some regularity. If there were no turbulence in a river, current speed would be uniform and we’d have a difficult time figuring out where trout might be. But turbulence slows the downstream energy of the water, creates those pockets of slower water that hold trout, and move the food supply in a stream in predictable alleys that trout take advantage of.
Don’t confuse laminar and turbulent flow with steady or unsteady flow. Whether flow is steady or unsteady looks more at the macro level of moving water. Hydrologists classify steady flow as water that is mostly all moving at the same speed and direction, with no change in depth. There is still some turbulence present, but it isn’t strong enough to change the average velocity or direction of the current. Unsteady flow is when water passing a given point moves in many directions, the velocity changes from one moment to the next, and the depth also varies when boils of turbulence raise the level of the surface. Trout will feed in both steady and unsteady flows, depending on the speed of the current and the amount of unsteadiness.
How Much Turbulence Is Too Much?
Slight to moderate turbulence is a good thing for trout. It reduces the downstream velocity of the water, so a trout uses less energy to stay in position. The little eddies and swirls of moderate turbulence may help a trout hold its position, and if you watch a trout at rest it’s seldom completely at rest, swaying its body in the current as it constantly adjusts but with a minimum of effort. At a certain point, though, turbulence becomes a liability.
Trout prefer predictability, as it’s easier to capture an insect drifting in the current when a trout spots its prey and makes a slight move to intercept it. Anything that suddenly veers off a path requires more energy to chase. In addition, if turbulence becomes too strong, it can push a trout back and forth, requiring more overt adjustments than just a slight body movement.
By nature, as we’ve seen, turbulence is chaotic and unpredictable. How much is too much, and what degree of turbulence will make trout avoid it? From what I’ve observed, if the cells of the turbulence approach the body size of a fish, it will avoid those places. A trout will chase a streamer into very turbulent water and occasionally even dart into it for an especially appealing nymph or dry fly. But it’s a low-percentage place and you’ll seldom find trout lying in areas of strong turbulence.
It seems to correspond to body size as well. In small mountain streams with trout seldom longer than a hand span, turbulence cells longer than about 6 inches in diameter are places these little fish avoid. Larger trout seem to be able to handle larger eddies of turbulence, though. Fishing the lower King’s River in California on a bright day with clear water, I noticed large rainbow trout hanging suspended and feeding subsurface over very deep water, in midwater where you would not normally expect them. I watched them for a while, trying to figure out why they were hanging in a place I wouldn’t expect them, and noticed that the strong turbulence at the head of the pool extended down into the center of the pool where I saw the fish. The cells were so large and, for the most part, repeated in a predictable manner, that the fish seemed to be riding a plume of current like a gull hanging in the air over a seashore, just on the upper boundary of the turbulence cell. They did sway more than normal and needed to adjust their positions with the vagaries of the current, but they seemed to be quite content feeding in the turbulence. When I’ve noticed trout hanging in strong turbulence they have invariably been rainbows, and I’ve seen steelhead behaving like this as well. It’s rare to see browns or brookies or cutthroats behaving like this.
There’s another reason trout avoid more-chaotic areas of turbulence. It’s often accompanied by dense air bubbles, such as the white water you see at the head of a deep, fast pool. Although that water is less dense and you would think trout might prefer it, I think they avoid it because they can’t see their food. The chaotic current combined with low visibility makes it a tough place for trout. However, these places also make a great spot for trout to hide because the reduced visibility makes it harder for predators to see them. I’ll often fish through a pool and then spook trout from under the boiling water at the head of the pool, but I think they bolted there when my bad casts and careless wading pushed them into the foam for protection. I don’t think they like feeding there.
At the other end of the scale are places where the surface is pocked with tiny goose bumps, as in a gentle riffle, or wavelets smaller in size than a trout that don’t propagate into bigger cells. This too is turbulence but of a more benevolent type. None of the cells are large enough to push a trout around, but the broken surface hides the fish from predators, and it hides gross mistakes in our presentation that smooth water wouldn’t allow.
Excerpted with permission from Rowman & Littlefield