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Home Fly Fishing Techniques How to Read a Trout Stream


How to Read a Trout Stream

    Picture of trout lie




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     As a fly fisherman, the single most important question you can ask is where are the Trout located within any give stream? However, in order to answer this question, you must first understand how to classify the different parts of a stream. For instance, have you ever been out on a trout stream and noticed that some sections of the stream are narrow and swift while other sections are wide and slow? Well, as fly fishermen, we have names for these different types of water and they are Riffles, Runs, Pools, and Glides and, under normal circumstances, the laws of stream hydraulics create these different sections in the order mentioned above. Thus, it is important for the fly fisherman to be able to identify each type of fly fishing water and to understand where the trout are holding (although the particular locations where trout position themselves are called “trout lies”, trout are said to “hold” there instead of “lie” there) in each section as well as how to properly present a fly to them. In addition, it is equally important that the fly angler be able to identify barren water versus productive water so that they do not waste their time drifting their flies over or through water where trout are not holding. Thus, understanding the anatomy of a trout stream is of paramount importance to understanding where the trout are likely to be holding and how to catch them. Consequently, it is absolutely imperative that you first learn to identify the various parts of a trout stream (riffles, runs, pools, and glides) and then, based upon a knowledge of the Food vs. Energy Equation and the three levels of the water column, that you be able to recognize which sections of the stream are best fished with a dry fly, a nymph, or a streamer, and then be able to identify each of the three types of trout lies contained within each section of the stream.



The Different Parts of the Stream -

     So, what constitutes barren water and what constitutes productive water? Well, first of all, barren water is any water that is to shallow to offer protection from avian predators and/or which has a bright, sandy, bottom that negates the Trout’s camouflage and thus, outlines him to predators. Productive water on the other hand is water that is 12 inches deep or more, has a dark bottom, and is either directly in, or adjacent to, the main current. Last, different sections of the stream have distinctly different characteristics and thus, fly fishermen have chosen to give each section a descriptive name such a Riffles, Runs, Pools, and Glides:


Riffles -

     What is a Riffle, where do the trout hold in a Riffle, and where do you drift your fly in a Riffle in order to place it in front of the Trout? Well, a Riffle is a section of the stream where the current is fairly swift but, the water level is fairly shallow and it flows over a bed of small, round, rocks or pebbles. Thus, the entire surface of a Riffle consists of small wavelets and mild white water. Consequently, Riffles are the aerators of the trout stream and because they hold the most dissolved oxygen of any section in the stream and, because they offer easy access to food, the entire riffle becomes a Prime lie if it is deep enough. Therefore, to fish a riffle, station yourself either downstream of or adjacent to and facing the riffle and then mentally divide the riffle into lanes about a foot wide. Then, cast your fly to the top of the first “lane” closest to you and let it drift for the entire length of the riffle (or as far as you can) and then, pick it up and recast it to the next lane over and let it drift. Then, you simply repeat this process until you have covered the entire riffle from side to side. Last, please note that it may sometimes be necessary to wade into the riffle in order to reach your next lane over which is fine as long as you do it SLOWLY. But, “what about the food vs. energy equation” you might ask? Well, Trout are anatomically designed in such a way that when holding in swift current against a flat bottom, all they have to do is place their lower jaw against the stream bed and the current with push them down and hold them there just like the wing in the rear of a race car. Then, in order to obtain food, all they have to do is tilt their Pectoral fins just a bit and the current will cause them to rise or descend though the water column.





Runs -

     Next, what is a Run, where do the trout hold in a Run, and how do you fish a run? Well, a run is a section of the stream where the current becomes very narrow, very swift, and is usually quite deep (although not always). Thus, because the current is much swifter in a Run than it is in a Riffle, the Prime Lies in a Run are adjacent to the current rather than in it. Therefore, look for large rocks either above or below the surface of the water as well as undercut ledges that will create eddies that provide the trout with shelter from the current but easy access to any aquatic insects drifting in the current. Then, drift your fly in the current as close to those Prime Lies as possible so that the trout has the least amount of distance to cover to seize your fly. In addition, Runs often extend into a Pool below them and thus, you will see a tongue of very swift water that extends into a body of much calmer water. Therefore, the edges of this current tongue are also Prime Lies and thus, you should cast your fly to the top of the current tongue right along the edge of seam between the swift water and the calm water and let it drift the entire length of current tongue.




Pools -

     Now, how do we define a Pool, where do the Trout hold in a Pool, and how do we present our fly to them? Well, a pool is defined as a small to large section of the stream that has a flat, calm, surface. Now, be aware that pools can be either very shallow, very deep, or anywhere in between but, they all have a (relatively) calm, smooth, surface. Consequently, this makes it much easier for predators to spot Trout in pools and thus, Trout have evolved super effective camouflage to prevent them from being detected when they are holding in calm water. However, if you take a dark colored object and place it over a light colored background, the dark object is immediately obvious because it is outlined by the light background and the same thing happens to trout when they swim over a bright, sandy, bottom in a Pool. Therefore, the Prime Lies in a Pool are going to be at the head of the Pool where any aquatic insects drifting with the current will first enter the Pool and along the edges of the current tongue that extends into the pool from the Run above it. However, if it is a large pool, there may be other places where the trout are holding such as any area with a dark bottom or a shadow from an overhanging tree (especially if it is strewn with varying sized rocks), behind or beneath logs that either extend into the stream from the bank or are submerged and are laying on the streambed, and along the banks under overhanging trees as long as there is enough current there to deliver a steady flow of aquatic insects.





Glides -

     Last, what is a Glide, where do we find the trout in a Glide, and how do we present our fly to them once we find them? Well, a Glide is essentially a Pool that is too long to be considered a proper Pool. For instance, picture in your mind your average, backyard, swimming pool and then, picture that same pool ten or twelve times longer and you will have the idea between a Pool and a Glide. Consequently, Glides are the most difficult of all Trout waters to fish because the surface is so calm and the water is usually deep enough that the trout have a fairly wide Cone of Vision and thus, they can see any fly angler coming from a long ways off. In addition, due to the calm current in Glide, the trout tend to cruise rather than hold (although this is not always true). Therefore, in order to fish a Glide, you will need a fast action rod with a light weight, floating, line and a long leader so that you can make long casts that will land gently on the water’s surface and enable you to stay out of the Trout’s Cone of Vision. Furthermore, rather than fish the entire Glide blind, stay on the bank use the intervening foliage to hide your presence as you slowly sneak upstream looking for cruising trout. Then, when you spot one, move back downstream beyond his Cone of Vision before casting your fly to him.






The Food versus Energy Equation -

     FYI, trout are first born as an egg and then, as they mature, they grow through different stages based upon their size called Alevin, Fry, Fingerlings, Parr, and Juveniles before becoming sexually mature and thus being classified as Adults. Consequently, once a trout completes the Alevin stage by completely consuming its yoke sack, the only two things on its mind from that point forward are avoiding predators and finding and consuming enough food to grow larger.


     Consequently, all trout look for quite spots in the stream called “lies” where they can escape the constant pull of the current and yet have easy access to food without unduly exposing themselves to danger from predators. Therefore, such places as eddies located behind rock or a log adjacent to good current flow make excellent “prime lies”. In addition, it is important to learn to identify prime lies because you will always find the largest trout in the prime lies. Now, the reason for this is a concept called the “food vs. energy equation” which states that, in order for a trout to grow larger, it must gain more energy from the food it consumes than it expends in pursuing and capturing that food. Thus, I like to think of the stream as a floating restaurant and the various aquatic and terrestrial insects drifting in the current as the menu and trout’s “prime lie” or “feeding lie” in the stream as their seat in the restaurant. So, when a tasty looking insect comes drifting downstream towards a trout’s “prime” or “feeding” lie, all a trout has to do is decide whether or not he wants to dart out into the current and capture that insect. However, it is important to be aware that there is also numerous bits of organic debris drifting in the current along with all those tasty insects and thus, a trout must become very adept at discriminating what is actually edible from what is not edible and they learn to do this by learning to identify the size, shape, and color of the available species of insects that inhabit or fall into their particular stream. In addition, they also look for signs of movement such as the gills located along the abdomen of May Fly nymphs as an indicator of life because any trout that mistakenly captures non-edible food is wasting energy. Plus, any trout that spends time swimming in the current to capture food is also wasting energy unless there is a tremendous amount of food drifting in the current such as when a May Fly hatch is occurring. Thus, when such hatches do occur, trout will often leave their sheltered “prime lies” and hold in the center of the water column in the middle or tail of a pool in “feeding lies” in order to more easily capture these nymphs and the emerging Duns. But, this exposed position also makes them far more vulnerable to aerial predators and thus, they are often very skittish when they are in “feeding lies”. However, the overabundance of food drifting in the current often makes the extra risk well worth the danger.


     Now, in addition to the various species of aquatic and terrestrial insects drifting the current, there are also numerous species of small fish inhabiting the stream along with the adult trout such as Dace, Sculpins, and Chubs, and Crayfish along with trout in the various stages of maturity, and all of them are fair game as far as adult trout are concerned. Thus, I like to think of these various bait fish in this manner: if someone were to offer you your choice between a free McDonald’s cheeseburger and a free 20 oz. steak dinner, which one would you choose? Personally, I would choose the steak dinner and most adult trout seem to feel the same way. So, although it obviously requires more energy to run down and capture a bait fish, that bait fish provides FAR more energy than consuming a couple of nymphs or dry flies. Therefore, trout will always consume the largest meal available as long as it provides them with more energy than they expend capturing that piece of food. Consequently, larger flies tend to catch larger trout.


But, it is also important to understand that when there is an overabundance of nymphs and duns drifting in the current such during a hatch, then trout will often become very selective in what they choose to eat for the duration of that hatch. Thus, they will often ignore any food item that is not the same size, shape, and color of the hatching insects because they have already determined through sampling that that particular insect is an edible source of food and thus it satisfies the food vs. energy equation.



The Three Levels of the Water Column -

     Something else all fly anglers should be aware of is the anatomy of the water column. So, first, imagine standing in the middle of a small creek while facing upstream and then, imagine slowly submerging yourself below the water’s surface. Thus, as you face upstream with the current flowing at you, the streambed would appear as a roughly “U” shaped cross section. Next, imagine the water column divided into three zones: the surface, the center, and the bottom. Now, where the surface of the water comes into contact with the air above it, the water encounters friction with the air that slows the surface layer slightly. Next, where the bottom of the water column comes into contact with the streambed, it also experiences friction with the streambed caused by gravel, rocks, and boulders that impede the waters flow and thus create turbulence in the form of both small and large eddies. Therefore, a layer of slower water is formed at the bottom of the water column as well that often extends a foot or two above the stream bed. However, since the center of the water column does not encounter any friction, it flows faster than either the surface or the bottom of the water column.




Anatomy of the Water Column

The Anatomy of the Water Column




     Now, the reason that this knowledge is important is that there are four different classes of flies that fly fishermen use (dry flies, wet flies, nymphs, and streamers) and  each of them is designed to be fished a different section of the water column. For instance, Dry Flies are meant to be fished on the water's surface while, Wet Flies and Streamers are meant to be fished in the center of the water column, and Nymphs are designed to be fished in the bottom of the water column. However, all too often, neither the water on the steam's surface nor the water below it flow at the same rate and thus they create disparate currents and whenever the water encounter obstacles, these obstacles create both turbulence and eddies. Consequently, this disparity in the swiftness of the surface of the water as well as the varying sections of the water column beneath it can inadvertently cause drag on your fly resulting in an unnatural drift that is not apparent to a fly fisherman but, which is very apparent from a trout's point of view; thus causing them to refuse to take your fly.


In addition, it is also important to remember that the bottom of the water column is where a Trout is most comfortable and thus it is where they spend the large majority of their time. The reason for this is the layer of slower water that exists there which allows them to expend little or no energy to maintain their position in the stream in addition to the abundance of aquatic insects that live among the substrate. Consequently, Trout are far more inclined to feed within their comfort zone but, they will rise through the water column to take a morsel of food if the reward vs. risk is great enough to warrant exposing themselves. Therefore, fly patterns that are designed to be fished either just above the bottom of the streambed (nymphs) or in the middle of the water column (wet flies and streamers) tend to be the most productive. However fly patterns designed to be fished on the surface, even they are the least productive patterns most of the time, are also the most exciting patterns to fish because you get to see the Trout come up for the fly and because of the predatory anticipation and lightening-like reflexes required to actually hook the fish.



The three types of trout fly water -

     So, once you understand about the different parts of a trout stream, the three types of trout lies, the three levels of the water column, and the "Food vs. Energy Equation", you can then gain an understanding of why some sections of a stream are more conducive to dry fly fishing whereas, others are more conducive to nymph fishing while, still others are best fished with a streamer. Therefore, the experienced fly fisherman can classify each section of a trout stream by judging both the depth of the water and swiftness of the current and then measuring that information against the "food vs. energy equation" in order to determine what type of fly is best suited for fishing each section of any given stream.


Dry Fly Water -

     Because a Trout has to expend energy to rise through the water column to the surface of a stream to take a dry fly and then return to its lie, according to the food vs. energy equation, a Tout has to consume a lot of May Fly duns or adult Caddis Flies to gain more energy than they expend in obtaining them. Thus, most Trout will not rise through deep water or really swift water to take a dry fly unless there is a hatch coming off. Therefore, good Dry Fly water is characterized by a depth of not more than two feet with a current that is not too swift.


Nymph Water -

     Because Trout have to expend energy to fight the current whenever they leave their sheltered lies and venture into the current to obtain food, they must obtain more energy from that food than they expend in obtaining it. Therefore, according to most of the published fly fishing literature, Trout obtain somewhere between 70 percent and 90 percent of their food from the subsurface currents. Thus, good nymph water is one and one-half feet to five feet deep (deeper if you are willing to use a longer leader) with a swiftly flowing current such as in a rifle or the current tongue of a run. However, they also work well when fishing Glides.


Streamer Water -

     Streamer flies are designed to imitate specific species of baitfish that commonly live in a trout stream. Therefore, good streamer water is whatever type of water the specific species of baitfish you are imitating at the time commonly inhabits. For instance, Sculpins commonly inhabit the rocky substrate in the main current even though they are not particularly good swimmers. Black Nosed Dace on the other hand are excellent swimmers but, they are most commonly found in the eddies and quite pools adjacent to the bank. Crayfish, however, are most often found in calm pools. Therefore, in order to fish streamers effectively, you need to learn what type of water each baitfish species inhabits and then fish the appropriate fly in the appropriate water.



The Three Types of Trout Lies -

     Trout living in streams need three thins to survive: oxygen, food, and shelter. Thus, they seek out places in the stream called "lies" that provide them with all of these requirements. However, not all lies are created equal. Thus, it is helpful to divide lies into categories according to which requirements they meet. For instance, some lies will serve as feeding stations only, while others will only provide shelter from predators. However, the ultimate lie will provide both shelter and easy access to food at the same time. So, in order to be a successful fly angler, you need to learn to identify these "prime" lies and fish them first. On the other hand, you should learn to avoid areas of unproductive water because birds are a trout’s main predator and thus they generally avoid areas with a light colored bottom and especially patches of sunlight striking the streambed because light bottoms tend to negate their camouflage and sunlight casts a shadow that outlines the fish to a predator.



Feeding Lies -

     Feeding lies offer a fish easy access to food but provide little or no protection from predators. Thus, most of the time, a fish will only move into a feeding lie when there is an over-abundance of food drifting in the water column such as when an insect hatch “coming off”. A good example of a feeding lie is when fish are holding in shallow water at the tail of a pool.  If the water at this point is less than three feet deep, then trout are vulnerable to avian predators and thus, they will only hold here if the availability of food in the drift is so abundant that it overcomes their instinctual paranoia. Another good example of a feeding lie is when fish are holding in mild current just under the surface of the water sipping Emergers and Duns. Thus, because of the way that their vision woks, they have a very small window through which they can see the surface world and thus they are again vulnerable to avian predators.




Feeding Lie

Feeding Lie



Sheltering Lies -

     Sheltering lies offer a trout protection from predators and thus they are never far from a sheltering lie that they can run to when spooked. However, they do not offer easy access to food so trout only move into sheltering lies when they feel threatened. Good examples of sheltering lies are crevices between rocks, ledges beneath rocks, underneath submerged logs, and inside sweepers.




Sheltering Lie

Sheltering Lie


Prime Lies -

     Prime Lies are the ultimate lies because they offer a trout easy access to food and they provide protection from predators at the same time. Thus, Prime Lies are prime real estat and therefore, the largest fish in any given section of a stream will always be found occupying the Prime Lies and thus, Prime Lies should always be fished first! Therefore, to identify a Prime Lie, you should look for a place with a dark bottom, a place with deep enough water or, with enough overhead cover, to provide protection from avian predators and, a place with a current that is not too swift and which provides easy access to the main current while also providing shelter from it. Consequently, as a fly fisherman, you will need to become adept at judging the color of the bottom, the depth of the water, the speed of the current, and the availability of cover in order to identify the Prime Lies in any given section of any trout stream. However, good examples of Prime Lies are the head of a pool next to a water fall, in an eddy behind a rock in the main current, pockets in rocky streambeds positioned directly under the main current, depressions in moderate riffles, underneath overhanging foliage adjacent to the shore, and underneath undercut banks in the elbows of bends. So, whenever you approach a trout stream, you should always plan to fish the Prime Lies first unless you observe fish holding in Feeding Lies. But, even then, you should still consider fishing the Prime Lies first because they may hold fish that are too large to engage in drift feeding.



Prime Lie

Prime Lie



    Consequently, the process of reading a trout stream consists of first learning to identify and classify each section of the stream and then learning to differentiate between barren water and productive water. Next, using a knowledge of both the anatomy of the water column and the food vs. energy equation, the fly fisherman must learn how to determine whether each section of the stream is best fly fished with either a dry fly, a nymph, or a streamer and then, he must learn to identify the Prime Lies located within each section so that he can present the correct type of fly to the trout holding in those lies if he is to become a successful fly fisherman.




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