Gary Borger fishes the tailout of a New Zealand pool.

Pool Morphology

In classic stream morphology, riffles/rapids and pools alternate, leap-frogging their way to the sea. Riffles are shallow, have coarse bottom sediments (usually rocks), and have relatively swift moving water. Pools are the opposite in all regards: They’re deep, have more fine bottom sediments (sand, silt, mud), and have relatively slow moving water. In fact, by definition, pools are the deepest areas in a stream. Riffles are easy to read because the swift, shallow water forms standing waves above the larger bottom objects. But in pools, the combination of slow currents, greater depth, and finer bottom sediments creates a rather uniform, smooth surface that reveals little about potential holding lies. However, with just a bit of understanding about the overall structure of pools, the fly fisher can quickly be fishing these waters.

Figure 5.1. In the classic pool, the head is fed by a riffle. The fast water extends into the pool a ways to form the throat. Reverse currents form on either side. The deep belly of the pool feeds into the tailout of the pool that dumps into the riffle below.

All pools can be divided into four zones (Figure 5.1). Zone one is the head of the pool. It’s the areas where the currents slow dramatically as they tumble out of the riffle into the depths of the pool. Zone two is the belly or body of the pool. It’s the slow, flat water in the pool’s center and is usually the deepest area of the pool. This is a great refuge for fish, and one can find them here almost anytime. The deep slow waters are a great place to fish a sculpin imitation on a sinking line with a slow, hand-twist retrieve. Nymphs fished on a long leader with a suspending indicator will produce fish in zone two. And on occasion, I have had great dry fly fishing in the belly of the pool when the surface has been alive with hatching or egg-laying insects. Zone three is made up of the edges of the pool, which are often shallow, but perhaps not, as we shall soon see. Edges are not places to be neglected, though many fly fishers view then as only a convenient wading zone. Zone fouris the tailout. It’s the downstream end of the pool where the currents gather themselves for another wild ride through the riffles or rapids below. The tailout is usually the shallowest area of the pool proper.

Zone One: The Head

The head of the pool usually displays three distinct features (1) the throat, which represents the currents that rush in at the top. The throat extends downstream to the point where the currents flatten out. (2) One or two reverse currents that form at the side(s) of the throat. (3) The transition in depth from the shallower water of the riffle/rapids to the deep waters of the pool, caused by the change in the profile of the stream bottom. There is often a rather abrupt change in depth, going from perhaps a few feet deep in the fast stretch to several more feet deep, quite quickly. This abrupt transition in depth is called the “lip” of the pool.

The Throat

The throat of the pool can have a variety of configurations, The currents that rush in may be fast, extending the throat a fair distance into the pool’s belly. They may be slow, and extend only a couple of yards before the surface flattens. The throat might be very narrow or quite broad, of great depth, or very shallow. Lots of possibilities, so potentially an area of feeding lies or prime lies, depending upon water depth.

Figure 5.2. The throat extends a ways into the pool, depending upon the speed of the water entering from the riffle or rapids above. Here, my friend, Mike Allen, is fishing the edge of the throat (dashed line) in a New Zealand stream.

Of course, fishing along the bottom in the throat currents can be effective, especially at the edges of the strong central current tongue where it interacts with the slower, usually reverse currents, at the sides (Figure 5.2). During a hatch or egg laying period, there are often fish feeding at the film in these areas.

Often times, in the big pools of steelhead rivers, the throat is an excellent place to find parked fish. They can hang in the highly oxygenated water, along the bottom and out of the currents, and prep for the next leg of the journey. But the throat is not just for staging steelhead. Any and all anadromous species will hang in bottom currents of the throat as a staging area (Figure 5.3).

Figure 5.3. The “soft water” just to the inside of the throat is a great place to find resting steelhead.

The Lip

The head of the pool is the area of greatest food influx because it catches all the drift from the riffle or rapids above. Thus, fish will often move to the head of the pool to feed. The fish hold in the throat and just downstream of the lip, under the fast water rushing in from the riffles (Figure 5.4). In this position, they can stay out of the heavy currents, but are still within striking range of any drifting food items. If the water at this position is less than knee deep, then this area is a feeding lie. That is, fish will go there to feed, but there is not sufficient cover to keep them there once they’re done feeding.

If the fast water just upstream of the lip is more than knee deep, then this area is a prime lie. And it’s one of the best the stream can offer. It has both food and protection from predators, and fish will hold there continually. These are the places you need to seek out, because they concentrate the fish, and there will usually be some in feeding mode.

Figure 5.4. The lip at the top end of the pool can be a prime, prime lie.

Jason and I spent a couple of weeks fishing for huge browns on Russia’s Kola peninsula and shooting a couple of TV shows. Filming is always difficult because of weather, the amount of gear that has to be dragged around, the need to get into fish quickly, and so on. On one of the rivers the chopper put us down next to a long, lake-like pool. Rather than waste time fishing the center reaches for the occasional fish that might be cruising there, we marched right to the head and immediately began taking fish at the lip—big fish, too, I might add.

If the water at the head position is more than waist deep, then this area will be a good nymphing or long fly area, but not usually a good dry fly area. The fish will not usually come up through all that fast water to take a tiny surface morsel. Then, too, it’s hard for them to see a tiny object that far away in all that fast chop. However, if there are big insects (like Hexagenia mayflies or big stoneflies), or maybe a mouse on the surface, then the fish will put out the energy needed to get a big mouthful like that.

Reverse Currents

The head of the pool is also the place of reverse currents. These cyclonic sweeps of water may be only a foot or two across in a small stream, but can be a hundred feet of more across on huge rivers (see Figure 5.1).

Food items trapped in these reverses stay trapped for long periods, and so a reverse can concentrate food. During a heavy hatch, for example, a reverse current can be literally jammed with the bodies of floating insects. Well, where food is concentrated, the fish concentrate. One of the rules of reading waters is that “Fish Follow Food.” So anyplace that concentrates currents, concentrates food, concentrates fish.

If the reverse is big enough and deep enough, it will hold fish. Less than knee deep the reverse will be a feeding lie; more than knee deep, it will be a prime lie. Always give such places a careful look before moving on. In fact, give them more than a look; fish them hard.

Reverses can set up anywhere there are current differentials. One finds them in riffles, rapids, and runs, too. Typically in riffles, the reverses are simply small swirling currents at the edges. They can suck insects off the main current and provide a food chain for feeding lies.

Excerpted from Gary Borger’s Reading Waters, Fly Fishing, the Book Series, Volume Two