THROUGH THE FRONT WINDOWS of the fly shop, I could see the motor home turn off the highway, crunch across the gravel of the parking lot, and roll to a stop. After a minute or two, two tourists, a man in his seventies and a boy of about ten, emerged, walked hesitantly throught the open door, and made their way to the front counter. The old man stuck his chin out slightly and looked across the counter at me.

"We're not from around here, and we don't know much about fly fishing, " he said, "but the boy here is set on catching a trout. I'd like you to sell us one or two flies and tell us how to fish them. They tell us there's good trout fishing here."

A tough assignment, I knew, but one that is handed out regularly in fly shops—guess the skill level of a customer, narrow a general fly selection down to one or two patterns, and condense an instructional guide trip down to a minute or so at the fly bins. I started my interrogation with the most important question, if I were to garner enough specific information to help them with selecting flies and suggesting fishing technique.

"So where exactly are you planning to fish?"

The old man looked at me, a puzzled expression on his face. "Oh," he replied, "right here in Montana."

Montana's wealth of trout streams show an incredible variety, from tiny mountain rivulets to rivers the size of the lower Clark's Fork or the Yellowstone between Big Timber and Columbus. Becoming a Montana trout expert by fishing every stream might sound intriguing. Even if fishing all of those thousands of miles of trout water were physically possible, it would turn recreation into drudgery, but there is a better way. Making sense of this infinite variety depends on the angler's ability to categorize it into different water types.

There are two sets of categories that determine how we fish a particular piece of water effectively. One division is defined by the source of a stream: spring creeks and tailwaters on one hand, and freestone streams on the other. Water source has important consequences for angling because it determines the kinds and numbers of foods that we imitate with our flies, as well as the most effective range of techniques.

The second set of categories, what most anglers think of when we discuss water types, is based on the slope the stream is moving across, because stretches of water that reflect varying combinations of current speed and depth will call for different techniques of presentation. In the 1970s, West Yellowstone author Charles Brooks taught a generation of anglers about these water types, and his terms for them—from very fast cascades and rapids, to fast water runs and riffles, and finally to slow moving pools and flats—are the most sensible to have appeared in print.

Spring creeks, as the name implies, receive most of their flow from perpetual springs. This water source results in a stable environment, nearly constant both in flows and water temperatures. The geology that results in such springs reaching the surface often coincides with alkaline water chemistry, an ideal environment for aquatic insects and crustaceans, favorite foods for trout. Hard water also fosters rich growth of aquatic vegetation which provides cover for the fish. A stream with a rich food supply, lots of cover, and stable conditions will support very dense populations of trout. The presence of lots of fish—and lots of bugs to imitate with artificial flies—makes a spring creek a mecca for fly anglers. Some of the best in the world are found in Montana: Armstrong, Nelson's and DePuy's in the Paradise Valley, O'Dell in the Madison drainage, Big Spring Creek near Lewistown, Thompson and Benhart's in the Gallatin Valley, Poindexter Slough, and numerous smaller streams.

Tailwaters are rivers whose flows are drawn from reservoirs. If the water is drawn from the cold layers at the bottom of the reservoir, and the flows are kept fairly stable, the result is a huge artificial spring creek. The numbers and size of trout that can be supported by a tailwater are staggering, as anyone who has fished the Bighorn or upper Missouri or Kootenai can attest.

Most spring creeks and tailwaters are found in the lower reaches of river valleys, where the gradient is not terribly steep. This means the water types encountered are not that varied—typically, a succession of gentle riffles and flats. The slow water allows for the deposition of smaller gravel and silt, and this fine material harbors mostly small insects. Another general rule with spring creeks and tailwaters is that, although they produce huge numbers of insects, there are usually a few hatches of certain bugs that predominate over the course of the season. The fish respond to having lots of the same food on the water for long periods by becoming "selective", that is, eating one food at a time, almost to the exclusion of all others. Contrary to popular fishing clichés, selective fish are not showing any kind of intelligence. Selectivity is just a natural response to a heavy and predictable supply of food.

Given this broad characterization of spring creeks and tailwaters, several observations about fishing them can be made. With fish feeding selectively to heavy hatches of small insects in slow, clear water, the angler is forced to fish small, highly imitative flies. The fly also needs to be presented in a very lifelike manner, with little or no drag in the drift. This usually means fishing with fine leaders—6X and 7X tippets are the order of the day. To some, this "technical" fishing with tiny flies and delicate tippets is the ultimate challenge in fly fishing; to others, it's strictly for sissies. Try it, and decide for yourself.

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