How to Read a Trout Stream

     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 the different the different sections of a trout stream which we call Riffles, Runs, Pools, and Glides and, under normal circumstances, the laws of stream hydraulics create these different sections in the order mentioned above.

     Thus, in order to be successful, it is important that a fly fisherman be able to identify each type of water and to understand where the trout are holding in it 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.

     So, what constitutes barren water and what constitutes what constitutes productive water? Well, first of all, barren water is any water that is too shallow to offer protection from avian predators 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 deep enough to provide protection from predators, has a dark bottom, and is either directly in, or adjacent to, the main current. Furthermore, as mentioned above, different sections of the stream have distinctly different characteristics and thus, we fly fishermen have chosen to give them descriptive names such a Riffles, Runs, Pools, and Glides.

     But, 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 (see picture below).

     Consequently, Riffles are the aerators of a trout stream and, because they hold the most dissolved oxygen of any section in the stream while also offering easy access to food, the entire riffle becomes a Prime lie provided that it’s deep enough.

     Next, we have Runs and thus, 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 directly in it. Therefore, you should look for large rocks either above or below the surface of the water that will create pockets and eddies in the current that provide trout both shelter from the current and easy access to any aquatic insects drifting in the current. 

     In addition, because Runs are usually followed by Pools, you will often see a Run extending into the Pool below it. Therefore, it is important to note that the edges of Run as it enters a Pool are also Prime Lies because they provide easy access to food while also providing shelter from the current.

     Next, we have Pools so, 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 a stream that has a flat, calm, surface (see picture below). Also, be aware that pools can be either very shallow or very deep or, anywhere in between but, they are all characterized by 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 or waterfall 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 rocks of varying size), behind or beneath logs that either extend into the stream from the bank or are submerged and are laying on the stream bed, and along the banks under overhanging trees as long as there is enough current there to deliver a steady flow of aquatic insects.

     Last, but not least we have Glides and thus, how do we define a Glide and, where do we find the trout in a Glide? 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 an idea of the difference between a Pool and a Glide (see picture below).

     Consequently, Glides are the most difficult sections of any trout stream 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 angler coming from a considerable distance. In addition, due to the calm current in Glide, trout sometime choose to cruise rather than hold (although this is not always true).

     Last, it is extremely important for all novice fly fishermen to become adept at identifying barren water. Thus, barren water is any section of a stream that does not provide sufficient shelter from predators or sufficient access to food to satisfy the Food versus Energy Equation such as extremely shallow Riffles, shallow Pools with bright, sandy, bottoms and, shallow Glides with slow moving currents.

     Furthermore, it is imperative that novice fly fishermen learn to determine where the Prime Lies are in each type of a trout stream and how to effectively present their fly to any tout holding in those Prime Lies if they are to become a successful fly fisherman.

     Consequently, learning how to read a trout stream while also learning how to present a fly to any trout holding in the various sections of a trout stream are truly quintessential skills that all fly fishermen must develop in order to become successful at this ancient and most fascinating form of angling. However, it does require keen observation as well as a firm understanding of how the structure of the stream bed affects the stream’s current as well as a firm understanding of the Food versus Energy Equation in order to understand how these factors affect where trout choose to hold in any section of any trout stream.

Written by,

Bill Bernhardt

Outdoor Professional

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