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
0'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.

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.

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.

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