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Rich and Poor Trout Streams

by Tom Rosenbauer
Geology determines the entire character of a trout stream. A glance at the surrounding terrain can tell you how big the trout will grow, how much food is available to them, and how they will be distributed in the stream; it also helps you predict their feeding behavior and even what flies will work.
Prospecting for Trout

Ed Jahn fishing Manchester Brook in August low water. I took him there only when I found he was leaving town.

LET ME TELL YOU about a couple of my favorite trout streams. One is Armstrong’s Spring Creek, on the O’Hair Ranch in the Paradise Valley of the Yellowstone River in Montana. You stop at the ranch house and pay an obscenely low fee (fifteen dollars in April, thirty during the summer) to fish one of the world’s richest little trout streams, in both insect life and trout population. You can wade across its transparent riffles and barely get your ankles wet, yet every day of the year a trout over twenty inches is a possibility on a small Pheasant Tail nymph or Blue-Winged Olive dry.

The other stream flows right through my town; let’s just call it Manchester Brook (loose lips crowd small streams). Manchester Brook flows with about the same volume of water as Armstrong’s Spring Creek, and it is about as wide, yet the largest fish I have taken in fourteen years of fishing it was just shy of thirteen inches. Where every pool in Armstrong’s holds scores of rainbows and browns over fifteen inches long, with many more small fish as well, a decent pool in Manchester Brook will offer one brook trout of ten inches, maybe an eleven-inch brown, and a half-dozen more trout of both species that, laid end-to-end, might total a couple of Armstrong’s average fish.

In Manchester Brook you can blind-fish with a Hare’s Ear nymph or the buggy dry fly of your choice: Humpies, Irresistibles, Haystacks, and Ausable Wulffs will catch trout all season long, any time of day. Yet if you try the same tactics in Armstrong’s, you’re guaranteed to draw a blank. In Armstrong’s you have to go two or three fly sizes smaller, and the successful flies will be of a different type from those you can get away with in Manchester Brook. If you try to prospect or blind-fish with a dry fly in Armstrong’s, you’ll spend much of your day looking at an unmolested fly. In general trout in rich streams won’t come for a dry fly if there is nothing of interest hatching, while trout in infertile streams will come to a dry almost all day long, even if there are no insects on the water’s surface. The difference between the two is geology, and nothing more.

Geology determines the entire character of a trout stream. A glance at the surrounding terrain can tell you how big the trout will grow, how much food is available to them, and how they will be distributed in the stream; it also helps you predict their feeding behavior and even what flies will work. Trout streams are made from water and rock, or rock particles like sand and silt. The chemical composition of the water comes from compounds leached from surrounding rock. These compounds encourage or discourage the growth of algae, diatoms, insects, crustaceans, and rooted aquatic plants, which form the food chain that supports a trout population. Surrounding vegetation also contributes to the food chain, as aquatic invertebrates feed on dead and decaying shore plants, but even this part of the chain is dependent on the geology of the stream’s banks. The slope of a stream, which dictates its riffle­to-pool ratio, is a function of bedrock. The types of rock determine the size of the particles in the bed of the river, which in turn fixes not only the number and type of aquatic invertebrates but also the number of places trout have to live.

Prospecting for Trout

Armstrong's Spring Creek. It has about he same volume of flow as Manchester Brook in April, and probably ten times the trout population. John Holt photo.

If you intend to fish only to rising fish during hatches, geology and a knowledge of stream reading are unimportant. You need only sample the drift to find out what flies will work, and you know where the fish are because you can see them feeding. But when you prospect without the benefit of hatches, you need other clues to help you select flies and find fish. The relative richness of a river, which you can usually determine with a few minutes of observation, is one of the most important clues.

Manchester Brook begins high in the Green Mountain massif, which is composed of Precambrian gneiss and quartzite. The metamorphic rocks that make up its bed offer little enrichment because these rocks are mostly insoluble silica. The water is much the same as rainwater runoff — on the acidic side with little of the dissolved calcium found in richer streams. The stony, thin, acidic soil also encourages the growth of conifers, especially hemlock, which, as the early settlers of New England found, is a source of tannic acid. The water has the tea-stained look of tannin, which comes directly from the hemlocks and from humic acid formed in bogs by the decomposition of organic matter.

Now if you look at a geological map of the Paradise Valley, just north of Yellowstone Park where Armstrong’s Spring Creek flows, you’ll see a band of Paleozoic Madison limestone and dolomite. This band crosses the Yellowstone River valley just south of Livingston, exactly where the three famous spring creeks — Armstrong’s, DePuy’s (actually the lower end of the same spring source as Armstrong’s), and Nelson’s — flow out of the ground and into the Yellowstone. Elsewhere in Paradise Valley, where the Yellowstone flows through basement rock of gneiss, granite, and schist, the tributaries are stony, with wide channels that indicate frequent spring floods. In midsummer the stream channels are often dry, or nearly so. But where the Yellowstone cuts through the ten-mile-wide strip of limestone, the character of the feeder streams changes dramatically. The soft limestone bedrock is dissolved by the acidity in rainwater and groundwater, and pressure on the water table from the high mountains on either side of the river valley squirts water up through holes in the wormy bedrock, forcing water to the surface in artesian springs and making the feeder streams run bank-full throughout the season.

One immediately apparent difference between the two streams is stability. Manchester Brook rises and falls and rises again from rainfall to rainfall. Snow melting in the spring in the mountains above raises the water level to the edges of the banks and beyond, while during a dry summer the brook shrinks to a tenth of its former volume, only to rise to early spring levels with a summer cloudburst. On the other hand Armstrong’s is monotonously constant, and when I fished it during the unprecedented dry summer of the Yellowstone Park fires, I could see no appreciable difference in flow from wet late Aprils during years of heavy runoff. This difference in stability partly explains the difference in growth rates of trout, and it also helps you figure out which flies and techniques to use in the two streams. Armstrong’s never floods, so the concomitant loss of food supply, heavy mortality of young trout, and expenditure of a lot of energy by adult trout evading floodwaters don’t occur. In Manchester Brook heavy mortality of both the food supply and the trout population is a yearly occurrence. Where Manchester Brook’s water temperatures range from the mid-30s in winter to the mid­70s in summer, Armstrong’s seldom waver from the 50s even in the dead of winter, because the water comes directly from the ground, and groundwater reflects the mean temperature of its latitude. Trout don’t feed and grow when the water temperature gets below 45 or above 70, so the trout in Armstrong’s are eating and putting on inches in mid-January, when the fish in Manchester Brook are in suspended animation.

Prospecting for Trout

The Ausable in New York state is relatively infertile, and as a result most of its trout are small.

There are other factors that make streams running through limestone richer than those running through quartzite, sandstone, or gneiss. Even streams that are not spring-fed but that run through limestone or other calcareous rock are much richer than those that run through mostly silicate rock. Penn’s Creek in central Pennsylvania is a good example. It is hardly a model of stability — in fact, every time I have tried to fish this famous river, it has been chocolate brown and over its banks. Yet even though it does not have the rich, clear, weedy character of a spring creek, because it flows through limestone bedrock it is much richer in insect and crustacean life than similar-appearing, silicate-bedded streams.

Given smaller seasonal stability and runoff pattern, a “hard” water stream will be richer than one with “soft” water. Any rock composed primarily of calcium or magnesium carbonate will leach into water, thereby “hardening” the water and giving you a richer trout stream. The most important such rocks for our purposes are limestone, composed primarily of calcium carbonate, dolomite, or calcium magnesium carbonate, and marble, composed of metamorphosed limestone or dolomite. Many empirical studies have proven that trout grow faster and behave differently in hard water streams. A Pennsylvania study of three soft water and three hard water streams with similar drainage patterns showed that trout’s growth rate was directly related to specific conductivity, which is a measure of dissolved calcium and magnesium salts in the water. Another study, in England, proved that limestone stream trout have a much lower seasonal variation in diet than those from soft water streams, and they grow faster as a result.

Before you think we’re going off on a tangent far removed from learning to catch trout when there are no hatches, let’s take a look at how rich streams differ from less fertile ones, and at some practical ideas that surface as a result. Assume for the time being that soft water streams are not as rich as hard water streams — you’ll learn how you can eyeball the relative richness of trout streams.

Predicting a Trout’s Feeding Habits

Rich trout streams have a steady, constant food supply. At the height of the richness scale are spring creeks, which differ little throughout the world. The Le Tort in Pennsylvania, Armstrong’s in Montana, and the Test in England have virtually the same food supply as spring creeks in Argentina and New Zealand. Twelve months a year the fish feed on midge pupae, small Blue-Winged Olive nymphs, scuds, and sow bugs. I grew up fishing a small spring creek in upstate New York, and the fly box I used when fishing this stream has served me well on spring creeks elsewhere. Although, if I fish a spring creek in midsummer, I add a couple of ant and beetle patterns, for the most part I can use the same half-dozen patterns in November or April.

Prospecting for Trout

A "collector" on the lower Henry's Fork in Idaho, similar to those on the Missouri.

Medium-rich streams like the Beaverkill, the Madison, or the Battenkill lack the constant water temperature and water level of spring creeks, but they still offer trout an almost endless buffet. The difference is that the kind of food changes throughout the season, and a nymph that trout climb all over in late April may be ignored in July. If I planned to fish a medium-rich trout stream like the Deschutes in Oregon, a river that I’ve never seen but hope to someday, I would not have the same confidence in the contents of my fly box, and I’d have to read up on the river, hire a guide, or stop in to a local fly shop before I chose my flies.

Relatively infertile streams offer a small and inconsistent food supply. Think of the upland brooks or mountain streams you have fished — the bouldery kind common in New Hampshire, North Carolina, Vermont, Montana, or California. Or imagine one of those boggy, tea-colored streams punctuated with beaver ponds that run through the lowlands of Maine, Michigan, and Wisconsin. Infertile streams are usually smaller — larger rivers run through wider valleys and pick up nutrients from rich bottomland sediments. If a larger river runs through rocky canyons, however, it too may offer a sparse food supply. Examples would be the Gallatin in Montana, the Ausable in the Adirondacks, or the Penobscot in Maine. It can be argued that the Ausable or the Gallatin offers good hatches for the fisherman, but the day-to-day food supply, the stuff that puts inches around a trout’s waist, is not as abundant as in rivers that flow through more fertile valleys.

The bad news is that you’ll have trouble predicting what kinds of food are prevalent in an unfamiliar infertile stream. The good news is you probably won’t have to. Further good news is that the flies you can get away with will be larger. Trout in infertile rivers don’t have the luxury of being selective, because they don’t see enough of anyone insect to get picky about which one they choose. Either they eat every piece of food that looks remotely edible or they starve. In most infertile rivers the quantity of aquatic insect larvae available to the fish by midsummer is insignificant, and they depend on terrestrial insects that fall into the water for a great part of their food. Since they never see many of the same kind of aquatic insects, and the terrestrials they feed on are a stew of all shapes, sizes, and colors (and we’ve seen in the last chapter that, all else being equal, trout prefer to eat the largest morsel of food available), all you have to do is turn over a few rocks or shake the bushes and decide what is the largest edible insect they are likely to recognize.

In more fertile rivers you have to pay greater attention to what’s on the menu. The trout are used to seeing multiple foods at any given time, and although they are not usually selective to a given species of insect, most of their food falls into specific parameters of size, shape, and color. If you go outside of that realm, you won’t draw as many strikes. Here the largest available food item might be rare enough that trout don’t recognize it. In the Battenkill, for example, most of the nymphs are small, skinny, and brownish olive-dull. If you turn over enough rocks, though, you’ll sometimes find a couple of those giant black stoneflies that trout go crazy over in the Rocky Mountains. I have tried size 6 stonefly nymphs in the Battenkill year after year, with never even a touch. Not only do the trout not eat them, I bet if I could look underwater I’d see them bolting for cover when that ugly nymph rolls into the neighborhood.

I’ve found that in richer rivers, smaller flies are more effective. I’m not exactly sure why. Perhaps it’s because smaller insect life is more abundant, and the fish are more likely to take a fly that’s similar to what they’re eating, while the fish in an infertile stream grab almost anything that looks edible. On the Beaverkill in mid­June, blind-fishing during the middle of the day when hatches were sparse, I once had to go down to a size 18 caddis to catch trout even in the riffles. Big Wulffs, variants, and other attractor flies didn’t even draw splashy refusals. I decided to explore a nearby tributary, which by the look of the water was nowhere near as rich as the Beaverkill. Tired of straining to see the tiny caddis, I put on a size 10 Ausable Wulff, more to enjoy watching the fly bouncing on the riffles than anything else. You know the rest of the story. In every pool there were a couple of trout eager to take the bushy fly as soon as it hit the water. This was less than a hundred yards from trout that wouldn’t even look at a fly larger than a 16. Since that day I’ve noticed that on rich streams like the Bighorn or the Battenkill, I seldom do well blind-fishing with a fly larger than size 16 (except for streamers and during grasshopper time). On small streams or on rivers like the Ausable or the Gallatin, for between-hatch periods I can get away with size 10 or 12 nymphs and dries — although during hatches of smaller flies I still use the tiny stuff.

One of the most important clues you can get from eyeballing the richness of a river is a sense of how the trout are distributed. When you don’t have the benefit of rising fish to tell you where they are, knowing where they should be saves you from fishing over unproductive water. There is nothing more frustrating than blind­fishing a piece of water, wondering if there are any trout at all underneath your fly. When I fish a stream I have never seen before and start to doubt the presence of trout anywhere near my fly, my confidence erodes and I lose concentration. As a result I can get sloppy about what I’m doing. If you know that feeling too, read on for a confidence booster.

The first time I fished the Missouri River was a lesson in the value of fishing water I would have passed up on other rivers. The Missouri is a productive tailwater, and its currents carry a rich soup of insect life all the time. Paul Roos, who was guiding my wife Margot and me, kept talking about looking for “collectors.” At first I couldn’t figure out what he was talking about, but Paul has guided on the Missouri for over twenty years, so I kept my eyes open and my mouth shut. When Paul finally pointed out a collector, I realized he meant the slow, barely swirling backwaters along the bank and behind islands in the river, where trout waited to collect flies that had drifted out of the main current. The trout could lie just under the surface without expending much energy because the currents were nearly imperceptible. Paul used the term ‘collectors’ to describe both the places and the fish, and we soon found ourselves gazing with intense concentration at water we wouldn’t have given a second glance on other rivers. The Missouri is so rich fish can thrive on the extra food that peels off from the main current. I suspect that on the Missouri the bigger fish are found in the collectors: their energy expenditure is at a minimum, so they can grow bigger, faster.

There were trout in the main currents as well, but because we were fishing a Trico hatch it was easier to spot the trout in the slow water. There is no reason to think that if we were fishing blind the trout would not have been in the same spots. Trout in rich rivers are evenly distributed, all over the place, because there is enough food to support them everywhere. Even in shallow sloughs with a mud or sand bottom, spots that look more suitable for minnows or frogs, trout can be found. In fact I’ve noticed that large brown trout in spring creeks seem to prefer these places over the deeper channels. On the other hand, in infertile rivers trout distribution is spotty. They will not be found in backwaters because it might be an hour’s wait for a piece of food to drift by, even at the height of a heavy hatch. So trout in rivers that aren’t so rich frequent the logical spots, the places that scream for a well-placed cast with an Adams or Hare’s Ear nymph. These logical places are the areas protected from the heaviest flow of water, but close enough to the main current so a sideways tip will allow trout to intercept food. At the edge of seams, at the tail of a pool, in front of and behind rocks, and where the head of the pool spills over a shelf-these are all logical places, and we’ll talk more about them in the next chapter on stream reading.

Applying Richness to Your Fishing Strategies

This knowledge, then, can help you form a fishing strategy. On rich streams, cover all the water. Never assume that a trout won’t be right in front of you, and concentrate on covering the water closest to you with repeated casts, changing flies or techniques often if you aren’t getting any strikes. Armstrong’s Spring Creek offers about a mile of water on the O’Hair Ranch, and they divide it among up to fifteen fishermen a day. One fifteenth of a mile of water seems like fishing in a closet until you get around a bend where you can’t see any other fishermen and you stare at the water. If the trout are rising or it’s sunny enough to see into the water, you won’t ever want to move, unless you must stretch your legs. I’ve often wished that someone would tie me to a cattle stile and make me fish twenty feet of water on Armstrong’s. I would be a better fisherman for the ordeal, and I would not be wanting for targets.

Prospecting for Trout

Trout in infertile rivers (top) will be found in the main flow, while those in rich streams (bottom) will be found throughout the channel, even in near-stagnant sloughs.

If you tied me to one of the hemlocks along Manchester Brook, though, I’d be ready to gnaw through the rope in five minutes. On infertile rivers, pass up much of the water, the stuff that doesn’t look fishy. Move faster between spots, then concentrate hard on the best­looking water. You can also move faster on infertile rivers because the fish don’t agonize over fly patterns — so neither should you. Trout in infertile rivers will move farther for a fly, so unerring casts are not as important here, and if your fly lands within a foot of where you think a trout is lying and floats drag-free (or swings properly if you’re fishing a wet or streamer), make a few more casts and move on. I don’t want to suggest that you get sloppy, but many times I have seen trout in unproductive streams move five feet for a dry fly. The only time a trout will move this far on a fertile river is when there are large, meaty flies like salmon flies (a huge, size 4 or 6 stonefly that hatches on western rivers) or grasshoppers on the water.

Remember I said I was going to give you some great excuses for getting skunked? Here’s one that relates to the richness of a trout stream: In fertile rivers trout appear to feed in spurts, with periods in between when they seem uninterested in any food and can’t be tempted with any fly. There are exceptions: although some biologists have observed these slack periods, Bob Bachman’s Spruce Creek fish never stopped feeding in the daylight hours when he could see them. Generally speaking, however, there are slack feeding periods in rich streams — not only in winter or the early season, when nobody argues with the fact that trout feed for only a couple of hours when the temperature climbs above 50 degrees, but during the height of the season, when water temperatures are perfect and insects are in the drift all day long. In streams that aren’t so rich trout feed even at high noon and in late afternoon (times when trout from richer waters most often take a siesta). Because they never get enough food, they are on the alert all the time. As we saw in the last chapter, though, trout learn to anticipate cycles of abundance, and trout in richer streams may be able to kick back for a couple of hours in the afternoon, knowing there will be a spinner fall in the evening.

If I’m fishing a biologically productive river like the Delaware or the Bighorn and I go without a strike for a couple of hours, I don’t brood, because I know he trout will switch on later. (This is assuming I have confidence in the fly I’ve tied on and the way I’m fishing it.) But if I fish over two pools in an infertile river without a strike, I look for another explanation. The fly I’ve chosen may be so far off that the trout won’t look at it or someone may have just fished through the pool and spooked all the fish. I may not be fishing the fly deep enough (often the case in high, cold water), or there may be no trout in these two pools. Another possibility is that I may have spooked the trout with clumsy wading and sloppy casts. In any case, if the situation arises, I either pack up and move, or sit on the bank and make adjustments to my tackle and my approach.

How Many Trout Are in That Stream?

The number and size of trout a stream can support are always limited by something, but almost never by fishing pressure or other predation. Populations are usually limited by the physical features of the stream, and you can make predictions about how many trout a stream holds by an estimation of its richness. Infertile streams have little migration, stunted adults, and many juveniles, Rich streams, on the other hand, are space-limited. Trout can get enough food anywhere in the stream, and the total number of trout is limited by the number of available places to hold and feed without wasting an inordinate amount of energy. A rich stream with a bottom covered with rubble of different-sized rocks offers lots of nooks and crannies to break the force of the current, and it can hold many more trout than a stream of equal richness with a sand or gravel bottom. A spring creek with many weedbeds offers protection from the current and places to hide when danger threatens, and it can hold more trout than an equally rich stream that has been widened, shallowed, and trampled by cattle.

Prospecting for Trout

Can you spot all the trout in this photo? A spring creek offers enough food to keep all of these fish happy, even though they're in close proximity.

A food-limited stream hosts trout of many different sizes, with frequent interactions among individuals (in competition for space) and net migration downstream. This migration, usually of the largest individuals in a population, can help you find some interesting fishing on today’s crowded waters. The lower reaches of many of our richest and most famous trout streams offer fishing for big trout in water usually thought to be the home of bass, northern pike, walleyes, and even carp. These lower-river trout seldom respond to hatches, if indeed there are any in such warm-water habitats, so prospecting techniques will help you find them. I’ve explored the lower Beaverkill and Delaware in the Catskills, well out of the famous trout water, and the lower Battenkill, and I’ve found surprisingly good fishing for large brown trout. Friends tell me about equally good fishing well out of the supposed trout range on the Madison, Bighorn, and Missouri.

Spawning conditions are poor in these places, and most of the trout ascend to the upper river to spawn, or they use a tributary stream. As you might suspect, water temperature is the main limiting factor for trout populations in these places. You can confine your search to the mouths of tributary streams, especially in the summer, and that is a relief because the lower reaches of these rivers are often huge. On one mammoth river in the Northeast (I would be risking the attention of a hit man if I used the name), there is a tight­lipped group of local fishermen who fish a deep, wide stretch of water a hundred miles below what is considered by the local chamber of commerce to be trout water. At the mouth of each cold­water tributary stream is a whirlpool, and just before dark these fishermen launch float tubes into the whirlpools and slowly revolve into the sunset. In a stretch of water known for walleyes and smallmouth bass, they catch rainbows that average about twenty inches long.


Aquatic crustaceans like this scud, show here with an imitation, indicate rich water.

Why Are Some Streams Richer than Others?

Now that you know some advantages to learning to gauge richness, let’s explore the reasons for these differences in productivity. Calcium compounds, found in many sedimentary and metamorphic rocks, counteract acidity. The amount of biomass in a stream, particularly the pounds per acre of trout flesh, is directly related to the pH. The more bicarbonate in solution, the more acid is neutralized, and the higher the pH. The higher the pH, the more productive the trout stream. Calcium in solution is also suspected to benefit the physiology of trout directly. Apparently it helps the fish fight off toxins in the water. Plants can also pull a molecule of carbon dioxide directly off calcium bicarbonate in solution, so streams with high calcium content support more plant life. More plants mean more insects. And fatter trout.

The amount of calcium in a stream also determines the supply of one type of organism that is extremely valuable as a trout food — crustaceans. The outer shell of these animals is made from a compound high in calcium, and because they absorb it directly from the water, the abundance of crustaceans in a stream is directly related to the. concentration of calcium bicarbonate. Crayfish, sow bugs, and amphipods (or scuds, as fishermen call them) are a high­energy source of food year-round. These animals do not hatch out of a river as aquatic insects do, so toward the season’s end and throughout the winter, when mature insects have flown away and mated and their offspring are too small to be of much use, full-sized, adult crustaceans are available. Crustaceans are easy to capture and high in protein and fat. Wherever they are found in great numbers, you will find lots of corpulent trout. Nymph fishing is superb in streams with large populations of crustaceans, to the point where trout often ignore heavy mayfly hatches because crustaceans give them a source of high energy without the costly risk of surface feeding. The chapter on nymphs will give you some ideas on how to take advantage of this opportunity.

A Brief Field Guide to Rich and Poor Trout Streams

Two minerals form the bulk of freshwater buffering systems: calcite and dolomite. Calcite is pure calcium carbonate, and dolomite is mostly calcium magnesium carbonate with various impurities. Limestone is the most common source of these minerals, and the most productive trout streams in the world flow through limestone. You can spot limestone bedrock by its sedimentary layers and its brown or yellowish color. Because limestone occurs in flat layers, rocks along the banks of a river with limestone are flat plates, as opposed to the rounded igneous or metamorphic rocks of less fertile streams. Sandstone and shale, sedimentary rocks that don’t contribute much to the fertility of a stream, can be distinguished instantly from limestone or dolomite with a couple of drops of vinegar or dilute hydrochloric acid. Rocks that contain calcite or dolomite effervesce or fizz when you put the weak acid on them. Did you ever think you could predict how to fish a trout stream by carrying a vial of vinegar with you?

Prospecting for Trout

These limestone ledges along the Madison in Montana betray its richness at a glance.

Gypsum and marble also buffer trout streams and make them richer because they contain calcium carbonate. You can tell them from other whitish rocks, such as quartz, because they lack the large, crystalline grains you see in rocks that contain quartz. They also usually have a crumbly look, which comes from their solubility in the weak acid of rainwater. Marble is metamorphosed limestone or dolomite, and because of the heat and pressure it has undergone, it is not as soluble as limestone (sometimes you have to pulverize it before it will fizz with weak acid), but marble still offers strong buffering properties. The Battenkill flows through a valley flanked by insoluble granite and gneiss to the east and marble bedrock to the west. Its pH fluctuates from around 5 below tributaries or springs entering from the east to more than 7 downstream of tributaries entering from the west. Trout in the brooks on the eastern slope will take a big dry fly all day long, regardless of the insects hatching or the time of day, but if you hop over to the other side of the valley, the trout often ignore a blind-fished dry fly. You have to use smaller flies that look more like the insects that are hatching during the current week, and the trout seem to have periods of lockjaw when no fly will work. The trout in the western tribs are also bigger and fatter.

Rocks composed mainly of silica contribute nothing to the productivity of a trout stream because they release no carbonates into the water. Silica rocks in streams can be recognized by their smooth, rounded shapes and crystalline structure. The ones you commonly see making up the beds of unproductive trout streams are gneiss, sandstone, quartzite, and various forms of granite.

Seldom do I predict the richness of a trout stream solely by staring at the rocks. It’s easier and more accurate to eyeball other clues in and around a stream and use the geology as one piece of the puzzle. For example, the color of the water can often be a dead giveaway to its richness. The tea-colored water so common in the north country indicates an infertile stream, where trout will be small, slow-growing, and eager to take almost any fly pattern. In the limestone belt of Pennsylvania many of the streams have a gray or white tint due to undissolved calcium carbonate, and the trout are well-fed, pickier about what nymph they take, and less inclined to come to the surface for a blind-fished dry fly. Water with no apparent color is not much of a help — it can indicate either a stream where all the brownish humic acid has been neutralized by carbonates, or, as in many high-altitude streams in the Rocky Mountains, water that has few dissolved minerals of any kind. Crystal-clear water can indicate purity, but absolutely pure water is less productive than water that contains some dissolved nutrients.

Civilized Richness

This is a hard pill for most of us to swallow, but water polluted with human or animal waste is always more productive than pristine water. H. T. Odum, one of the world’s leading ecologists, once wrote, “Polluted streams are possibly the areas of highest primary productivity on the planet.” The Bow River in Alberta is one example. Above the city of Calgary the Bow is relatively infertile and can be easily blind-fished. Below the city, where the waste of over a million people enters the river, it is fertile beyond comparison in that part of the country, and the trout show the pickiness, reluctance to feed at certain parts of the day, and hesitance to come to the surface that are common among well-fed fish. Studies in Michigan and Pennsylvania have shown that removing domestic sewage can dramatically reduce the productivity of a trout stream, while adding it can make an infertile stream rich. The same goes for water that flows through agricultural land. Sewage and agricultural fertilizer are rich in phosphates and nitrates, and the lack of these nutrients often limits plant growth in streams, so when you add them to a stream you get the same effect as when you sprinkle 5-10-5 on your sweet corn in the spring. A study in Wisconsin found that runoff from one hectare of agricultural land puts 7.7 kilograms of nitrate per year into a trout stream. This beneficial effect walks a fine line because pollutants can also increase the biological oxygen demand of a stream, especially in hot weather, and too much organic material without cool water and a lot of riffled water can suffocate trout.

Weeds in the water always indicate higher productivity, and as a result more invertebrates for trout to feed on. Watercress and stonewort thrive in alkaline environments rich in carbonates, and long, thin, bright green strands of filamentous algae tell you either that the water is rich in carbonates or that sewage or agricultural effluent is present. In a stream that runs through a town or city, you’ll often notice that the bottom of the river is clean above town, while below town the rocks have a coating of algae or long strands streaming from them. The water will be richer below town, as it is in the Bow, but you should be aware that not all rivers have the head of cool water to compensate for the increased oxygen demand during the summer.

You can predict the richness of surrounding trout streams by taking a shower in a nearby house or inspecting the owner’s plumbing. (You thought it was bad enough that normal people ridiculed you when you walked around in trout streams with a butterfly net. Now you’re going to be knocking on doors asking for a cup of vinegar and a look at the bathroom sink.) Calcium carbonate in the water is the same stuff that causes “lime” in your plumbing. If the local water supply contains a high concentration of carbonates, chances are the nearest trout stream does, too.

Prospecting for Trouit

Tailwaters like the South Platte can be as rich or richer than spring creeks, and the banks usually show stability, with no sign of frequent flooding.

Tailwaters Are Usually Rich, Too

I lied when I told you that spring creeks are the richest trout environments in the world. They are the richest natural trout environments. As a class tailwaters are the richest trout streams in the world, and when you think of the waters fishermen dream to wade in, you have trouble leaving out the Henry’s Fork, Madison, Bighorn, Missouri, Delaware, White, Green, South Platte, or Frying Pan. All of these rivers famous for their imposing trout and plentiful hatches are made rich by the still waters above them. Dams, if they release water from the bottom of the reservoir above them, as most of the famous ones do, stabilize both flow and temperature by being miserly with spring runoff and doling it out throughout the summer. Floods are reduced, temperature extremes are moderated, and growth is easier. Nutrients are concentrated in the impoundments behind dams. Trout also benefit in tailwaters because plankton is washed directly into the rivers and eaten by insects and crustaceans. Natural streams have little plankton because it’s hard to maintain a population if you keep getting washed downstream, so invertebrate life in tailwaters enjoys a tremendous bonanza found in few natural environments.

How Valuable Is Rock Flipping?

You might be thinking I’ve left out the most obvious way of determining the richness of a river — picking up a couple of flat rocks and looking at the insects waddling madly to get away. Unless you’re prepared to set up a seine, however, and trash a couple of square feet of stream bottom to get a representative sample, and then compare this sample to other streams, I don’t think you will get a fair idea of richness from rock turning. You may miss the right part of the riffle and pick up rocks that are barren just by chance. You might be looking late in the season, when most of the larger insects have hatched and their offspring are too tiny to be noticed. Many of the insects in a river cannot be found by turning over rocks — you’ll only find the clinging and crawling species and will miss the burrowers and swimmers. Sculpins, other forage fish, and crayfish are food chain supplements, ingredients that support big trout, but you’ll seldom see them when you turn over rocks in the shallows unless you look at the place the rock was rather than what is clinging to it. Also these animals usually live in deeper water than you want to reach your arm into.

Looking at rocks helps you pick a nymph pattern, particularly in richer streams where the pattern choice may be important, and although it can give you a hint at a river’s diversity, diversity is not as important as richness when it comes to working out a fishing strategy. Gauging the richness of a river is like “pre-stream reading”: a way of looking at the river as a whole system before you start gazing at current patterns and rocks.

MidCurrent Fly Fishing
Tom Rosenbauer has been a fly fisher for over 35 years and was a commercial fly tier by age 14. For 30 years he has been with the Orvis Company, where he is now marketing director for Orvis Rod and Tackle. He also has written ten fly fishing books. Excerpted from The Orvis Guide to Prospecting for Trout: How to Catch Fish When There's No Hatch to Match, Revised Edition (The Lyons Press, December 2000, 288 pages).
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