The book begins with some basic concepts of fluvial geomorphology and how different types of rivers form. It reminded me of many of the concepts I learned in an introduction to watershed science course I took back in 2009. The book also covers textually many of the topics that Wendell Ozofovich has illustrated in his underwater video series. I won't cover most of what I've read so far in this post but one specific concept and its application to nymphing piqued my interest this week.
Laminar flow occurs both horizontally and vertically in rivers. Friction with substrate reduces the velocity of water near the bottom and banks of a river. A small amount of friction at the air/water interface also slightly reduces the flow velocity near the surface. Velocity peaks in the water about 1/3 of the column below the surface. The faster the maximum velocity in the column, the greater the difference will be between the maximum and minimum velocity in the river. Furthermore, immediately adjacent to the substrate a boundary layer of water forms where the velocity is near zero. The larger the substrate is in the river, the wider the boundary layer becomes. In a gravel bottomed river, the boundary layer may only extend an inch or two above the bottom. However, in large cobble or boulder substrates, it may extend a foot or so above the substrate. The graph below illustrates these concepts of vertical laminar flow and a boundary layer in the river.
Trout often occupy the boundary layer as a means of energy efficiency. It is easy for them to expend little energy holding their position in the boundary layer and a simple tilt of pectoral fins is all it takes to elevate them into the column to capture a drifting food item. This is one reason why hatchery trout exhibiting fin erosion are at a disadvantage physiologically. To avoid venturing on a biological tangent, let me turn to how this boundary layer may affect nymph rigging.
First, in faster rivers where the velocity of the upper column is much greater than the velocity where trout are holding near the bottom, an indicator or thick leader diameter that catches extra drag in the faster upper column will induce downstream drag that lifts your flies out of the boundary layer and away from trout. Having only thin tippet penetrating the water will minimize the downstream drag influence of the faster upper column.
Second, on the terminal end of your leader, this hypothetical velocity profile has some implications for the number and arrangement of your nymphs. Though this post is tailored toward weighted nymph rigs, it has the same implications for unweighted nymphs and split shot. Let me cover the four basic possibilities below:
1.A single fly rig gives you the best chance to fish your nymph in the boundary layer at the level of the trout and with the most drag free presentation. It also allows the best casting accuracy and eliminates the issue of having flies landing in different current seams horizontally as well. However, in deep water you may need a large heavily weighted fly to attain sufficient depth. Trout may want a smaller offering especially in the late sessions of competitions or in waters that receive a lot of pressure from day to day anglers. You may have to add a second or third fly to attain the weight necessary to attain depth. Being limited to 1 fly at the World Fly Fishing Championships in Slovenia a few years ago made it difficult to catch fish in large deep rivers especially toward the end of the tournament when the fish had seen plenty of flies.
With a 2 fly rig you have the choice of 2. placing your heaviest fly on the point and a lighter/unweighted fly on the dropper tag above or 3. placing your heaviest fly on the dropper and a lighter fly on the point. Let me suggest some possible pros and cons of both.
With rig 2 (heaviest fly on point/bottom), the heavier fly will pull the leader tight and drift below the lighter fly above. In Fips Mouche competitions there is a rule that flies must be spaced at least 20" apart hanging vertically. Assuming that your nymphing leader approaches a somewhat vertical profile and that the boundary layer is only 10-12" or less, it is impossible to fit both flies of a 2 nymph rig in the boundary layer. This is represented in the figure below.
As a result, the dropper fly ends up above the level of the trout and succumbs to upstream drag because of the influence of the heavier fly in slower current below. However, there are reasons this configuration may still be valid. The straight and tight leader increases strike detection because it provides a more direct contact with your flies. In Dynamic Nymphing, George Daniel also suggests either a single fly rig or a 2 fly rig with a heavily weighted fly on the point and an unweighted fly above to fish in pocketwater or complicated currents. The heavy fly will fly will pull the rig into a single current seam from a horizontal perspective and minimize the occurrence of your nymphs drifting in to vastly different current seams. Your dropper fly would still be subject to the vertical drag I'm referring to however. Having weight on the end of the rig also facilitates tuck casting and vertical leader entry. Furthermore, in situations where fish are suspended feeding on emerging insects, a fly up in the column may improve success even if dropper fly is subject to drag simply because your dropper nymph will be at or a bit above the level of the trout.
With rig 3 (heaviest fly on dropper tag), the heavier fly descends to at or below the level of the lighter fly on the point. This allows both flies to enter and stay in the boundary layer as long as they are in the same horizontal seam of current.
Therefore it may create the best drag free drifts. However, this rig does not make tuck casting easy because the lighter fly tends to turn over horizontally beyond your heavier dropper fly. It also creates a hinge shape in the leader that must be straightened before strikes can be detected on your point fly. Furthermore, if the heavy dropper fly lands in a different current seam than the lighter point fly, the point fly will drag horizontally until it swings into the same current seam.
4. The last rigging choice is a 3 nymph rig. If it is hard to fit 2 flies into the boundary layer you can imagine how much harder it is to fit 3 flies into the boundary layer. Therefore, I only use this rig when I fish very large deep rivers where attaining depth is difficult, especially with small lightly weighted nymphs.
Hopefully from this post you can see there is a 3 dimensional challenge to attaining the correct properties of drift when nymphing. I think the crux of the matter I've taken away from reading Moving Waters thus far, and applying it to my time on the water, is that one way of rigging may not be the right way for every situation. I will continue to experiment with the effects of rigging on different water types and I challenge you to do the same.