Factors that make a trout stream
The quality of a trout stream depends on suitable streambed geology, adequate groundwater, and compatible land use in the watershed. A good trout stream is a lucky mix of these ingredients that can be improved only within the limits of basic productivity. Listed below are just some factors that determine the quality of a trout stream.
The gradient, or steepness, of the streambed influences the swiftness of the current. Current continually moves food through the stream system and mixes oxygen into the water. Slack water can be too warm to support trout. Conversely, continuous cascades and rapids make for poor trout habitat (though trout have adapted to swift water better than most fish). Between these extremes, however, trout can flourish, depending on other characteristics of the stream.
The makeup of the streambed, or "substrate," depends on the geology of the region and the velocity of the current, which sorts fine materials from coarse.
A mixture of boulders, cobble and gravel is best for trout and the invertebrates on which they feed. Silt provides a burrowing medium for some insects, though it provides little else to a trout stream. Sand is nearly lifeless; large amounts of it contributed by erosion cover more desirable bottom types.
The shape and composition of the streambed provide food and cover for trout. Gravel that is gently bathed by current provides spawning areas. Boulders and coarse substrate break the current, providing places a trout can rest while waiting for food to drift by. Deep pools and undercut banks provide refuge during sunny days and low water. Riffles are food factories, and trout often move into them, especially in low light, to actively feed.
Cool water, stable flows and oxygen
Temperature, more than any other factor, distinguishes trout streams from those inhabited by bass, walleye, northern pike, catfish and other species. Trout need cold water and cannot tolerate temperatures above 75 degrees for long. Because of our relatively warm summers, our best trout streams are fed largely by springs, which average less than 50 degrees and keep the stream cool in summer. The amount of dissolved oxygen in the water (and trout need a lot of it-about five milligrams per liter) depends also on turbulence. Riffles and falls reoxygenate water. The flow of groundwater is much more constant and reliable than the runoff of rainwater. Consequently, spring-fed streams remain filled during drought and are less prone to flood. Streams that depend solely on surface runoff tend to be "flashy," flooding after a rain and dropping soon after. These extremes stress fish, damage eggs and kill fry.
Alkalinity, hardness and pH
Though these terms refer to different qualities, they are closely related as they apply to water chemistry. These characteristics influence productivity-not only of trout, but also of invertebrates. Generally, the best trout streams are hard (plenty of dissolved minerals) and alkaline (dissolved calcium carbonate) and have a pH of 7.5 to 9 (not acidic). The most desirable of these qualities are found in spring-fed limestone creeks. By contrast, streams that depend on runoff and flow over volcanic bedrock are less productive because they are generally soft, neutral to slightly acidic, and low in alkalinity.
Moving water does not support the abundance of microscopic plant life that still water does. Consequently, compared to lakes, streams depend less on drifting phytoplankton for their food base, and more on leaves, stream side plants and other plant materials that fall or wash in from the nearby land. Productive streams support lush growths of aquatic plants.
Next in the food pyramid are the plant-eaters, detritus-eaters and scavengers, which range from microscopic invertebrates to four-inch crayfish. Many of these small crustaceans are important foods for trout and forage fish.
Of particular interest to fly-fishers are various aquatic insects-mayflies, caddis flies, stoneflies and several others-that live their early lives amid rocks or burrowed in sediment. Late in their lives, they crawl or swim to the surface and emerge as winged adults to mate and die. Trout feed heavily on the immature and adult forms of these insects. Trout often feed ravenously during an emergence, taking only the form of insect that is "hatching." The angler who can "match the hatch" with an appropriate fly can enjoy terrific fishing at these times.
Some trout streams are rich in invertebrates but lack forage fish. In these waters, trout rely on insects throughout their lives, though some large trout may also prey on other trout. In other streams, where forage fish are abundant, adult trout may feed much more heavily on fish.
A trout's life in a stream is a precarious balance of energy expended versus energy consumed. Fish that race energetically in pursuit of food would soon die. Instead, trout have evolved to occupy "lies" behind logs, against deep banks, in the upper ends of eddies, or nestled in coarse substrate. There they wait for current to deliver food to them. Large morsels-a four-inch dace, for example- may be worth great effort in chasing. But small items are worth small effort-a gentle rise to the surface or move to the side to sip the insect and then a smooth return to the lie. When a particular insect is emerging in abundance, the efficiency-minded trout may ignore all else to feed exclusively on, for example, emerging mayflies of a particular species. It is then that a stream may seem particularly alive, the insects rising to the surface like bubbles through champagne, and the trout sipping flies from the surface of the water, dimpling the stream as though a gentle rain were falling.
About 600 Minnesota streams totaling nearly 2,000 miles are designated trout waters (though habitat is marginal in some). Most of Minnesota's trout streams lie along the North Shore of Lake Superior or in the rugged hill country of the southeast.