The Anatomy of the Water Column: understanding trout mentality

This post was born 26 Mar, 2016 No comments

In the first installment of this series on trout mentality, I described to you how to think like a fish by describing the underwater world and the many dangers a trout faces on a daily basis throughout his life and, in the second article, I described what both the underwater and surface worlds look like from a trout’s perspective and explained why precision casting is so very important. Therefore, in this third installment, I will describe the anatomy of the water column and how it affect your fly as it drifts and, in turn, how it affects a trout’s feeding behavior.

Thus, the first step in understanding the anatomy of the water column is to once again imagine yourself in a stream. However, rather than viewing it from the trout’s perspective as we did in the second article on how a trout sees his world, in this article, we will examine it from a human perspective. Therefore, imagine that you are standing in the middle of the stream and facing the current flow. There you will see both banks of the stream as well as the many boulders and smaller rocks that protrude above the water’s surface. In addition, you will see the surface of the water agitated by these various obstructions as well as those beneath the surface. Now, imagine slowly lowering yourself down to the surface of the water and then immersing yourself below it. There, you will have a clear view of the stream bed as well as all of the water above it but, as I mentioned in the second article, the surface will appear as a huge mirror that reflects everything beneath it. Now, in order to understand how the water column works, it is helpful to divide the water between the stream bed and the surface in into three different zones consisting of the surface layer, the water between the surface layer and the streambed, and that which flows along the streambed itself. Thus, if you were to measure the speed at which these three layers are moving toward you, you would find that they are all moving at three different rates of speed and the reason for this is that as the water in the surface layer comes into contact with the air as it flows downstream, the friction between the air and the water causes the water on the surface to move somewhat more slowly than the water immediately below it and, as the water beneath the surface flows over the stream bed, it also encounters friction which causes it to move more slowly. Plus, as the water flowing over the stream bed encounters obstructions such as gravel, rocks, and boulders, it creates a layer of turbulence just above the streambed as well as numerous eddies in the center of the water column. Consequently, the water flowing between the surface layer and the streambed has the least amount of resistance to its forward movement except where it encounters the occasional obstruction and thus, it flows faster than the water in the surface layer and that which flows along the streambed.

Water Column Graphic

Now, the reason that this explanation is important to you as a fly fisherman is that fly anglers commonly use one of four different types of flies consisting of dry flies, wet flies, nymphs, or streamers and each of these types of flies is specifically designed to drift (or swim) in one of the three zones of the water column. For instance, as the name implies, Dry Flies are designed to drift on the surface of the water whereas, both Wet Flies and Streamers are designed to drift or swim in the center of the water column and, in most cases, nymphs are designed to drift along the bottom of the water column. But, in order to appear natural to the trout, each of these flies must drift through the water column without any apparent drag. However, when fishing dry flies on the surface of the water column, you will inevitably encounter surface currents that move at different rates than those adjacent to them and thus, if you cast your dry fly into one of the slower currents, the faster currents between you and your fly will cause your fly line to drift downstream faster that your fly which then causes the fly to be dragged along the surface which, in turn, cause a small, V shaped wake to appear behind the fly (called “boating”) which causes it to look unnatural to the trout and thus, they will ignore it as food source. Then, of course, if you are drifting a wet fly through the center of the water column, then the same surface currents which affect your dry fly’s drift can also adversely affect the drift of your wet fly which, once again, causes it to appear unnatural to the trout. On the other hand, if you are drifting a nymph in the slower currents near the streambed, then you have both the contradicting currents in the surface layer as well as those in the faster water immediately beneath it to contend with and thus, there again, these currents can cause your fly to drift unnaturally. Consequently, advanced fly fishermen have developed numerous different fly casting techniques such as the Slack Line Cast and the Pile Cast to enable them to contend with contrary surface currents when fishing with dry flies as well as the technique of mending the line upstream when fishing with wet flies and nymphs in order to cause their flies to drift naturally for longer distances; thereby increasing their chances of the trout deciding to seize their fly.

Therefore, because a trout must gain more energy from the food that it chooses to eat than it expends in entering the current to obtain that food (called the food vs. energy equation), trout necessarily become very adept at a very young age at determining what is edible and what is not. Consequently, it is imperative that all fly fishermen understand the of the anatomy of the water column and its three different zones so that they can also comprehend how a fly line laying on the surface of the water and/or extending through each of the different zones of the water column can adversely affect the manner in which a fly drifts by causing it to appear unnatural to the trout and thus cause them to reject it as a viable food source.

Written by,

Bill Bernhardt

Outdoor Professional

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