There are numerous types of trout flies to choose from as well as various strike indicators, different style fly lines and leader lengths. They all have different uses, but the first step is understanding how to choose a fly and utilize specialized gear based upon understanding different rivers.
While every river and stream will have its unique characteristics, all fall into one of three broad categories: Freestone, Spring Creek, or Tailwater. Each of these categories has distinct elements that determine the type of aquatic life found there. A river's water flows, water temperature, and fertility are all determined by its characteristics.
A freestone stream's primary trait is that its flow directly related to precipitation. The more rain or snow that falls directly translates to how much water is flowing in the stream. This precipitation also means that water temperatures will vary quite a bit as well since there is a direct correlation to weather and the amount of water in the stream. During winter you may find ice covering large pools and during the hottest days of summer water temperatures may get high enough to make trout move to find colder water to survive.
Freestone streams are typically rocky and very clear like Little River in the Great Smoky Mountains of Tennessee
A freestone stream is governed entirely by natural cycles and weather.
These natural cycles mean fishing on freestones has somewhat predictable periods of higher flows during spring rains or as mountain snows melt. Flows will moderate as the season progresses and warm as well. Water temperatures can be perfect in spring and early summer. Many years will see flows recede in summer and water temperatures climb. Water flow on freestones is usually at their lowest in autumn which is typically pretty dry in trout country in North America. Snowmelt is long gone, and groundwater tables are at their lowest. Due to winter weather being colder and wetter, a rebound to flows as water temperatures drop.
Since precipitation feeds freestone streams, these waters are relatively infertile compared to tailwaters and spring creeks. There will be quite a bit of variability because of local geography. The fertility of freestone streams is a result of pH and fertility of surrounding soil and geology. These streams are always the most vulnerable to pollution as they are the product of where the water flowed right before it entered the stream.
The variety of seasonal temperatures and flows allows for a range of aquatic life in these streams. Freestones will usually have high varieties of insects, but relatively low densities of them. A large variety translates into fish that may not be very picky about what they eat since they see many foods but relatively few of any one type. Aquatic life in freestones can be the polar opposite of aquatic life in spring creeks and many tailwaters where insect densities can be exceptionally dense, but variety may be rather low.
Freestones make up the vast majority of trout water, but in most cases, they are relatively small rivers or streams. In the eastern United States, most trout waters will be in the mountains where water remains cold enough to sustain trout. Most of the mileage of trout water is composed of freestones, but the largest, and in many cases most popular trout rivers are not freestones, they are tailwaters. These rivers have dramatically different characteristics from freestones.
The identifying characteristic that creates a tailwater is a human-made dam that controls the flow of water. In this definition, a dam is merely a static barrier across the stream that produces a large pool or reservoir. The dam will control the flow, sometimes holding back a massive deluge or allowing more water through. The resulting flow from a tailwater is determined by man, and in most instances is far different from natural cycles. Large rivers have been dammed to control floods and generate power. A dam that controls the flow of water has dramatic effects upon aquatic life and in many instances have either enhanced trout fisheries or created them where they could not exist before.
Dams are created for a variety of purposes, but the most common ones are to control the flow of water and the second is to generate electricity. Most dams can do both, but depending upon where you are, one will take precedence over another. In East Tennessee where we live, TVA (Tennessee Valley Authority) built dams on the Tennessee River, and it's tributaries to control spring floods, make river travel easier for barges, and generate electricity. Several of these dams on tributary rivers are quite large with very deep reservoirs. These lakes are so deep that water on the bottom of the lake is frigid and the reservoir of water so great that these rivers now run cold when they used to run quite warm in the summer. These rivers are unnaturally cold and reliably so.
If you've never been to one of these rivers, it's mind-boggling to arrive on the Clinch or South Holston River on a hot August day yet find a large river with water a bone-chilling 45 degrees. These are typical of winter water temperatures in Smoky Mountain trout streams about an hour's drive away. Not only is the water this cold in summer, but it will also very likely be pretty close to that temperature in the winter too. Trout, and more importantly aquatic insects, in freestones, must negotiate a much higher set of temperature conditions over the year than tailwater trout do here.
The most important part of the definition of a tailwater, though, is the controlled flow by the dam and whoever manages it. Another striking detail about some tailwaters, particularly those operated by TVA and the US Army Corps of Engineers, is that flows will change suddenly and dramatically to generate electricity. Flows on the Clinch River will regularly fluctuate between about 200 cfs (cubic feet per second) to 7,000 cfs or more and back again in a single day. Fortunately, generation schedules are posted for anglers to check, but anyone wading the river needs to evacuate before the water rises. This continual dramatic fluctuation of flow impacts the types of aquatic insects that can thrive.
An important thing to realize is that all of the TVA and Army Corps of Engineers tailwaters across the southeast, as well as many other tailwater trout fisheries, have been created. These were not trout fisheries before the dams, and now there are viable trout fisheries in as unlikely locations as Alabama and Texas where the natural climate is not conducive to trout.
Western tailwaters will usually generate electricity, but their primary function is to hold water for irrigation needs. As a result, flows will vary according to the needs of agriculture and other necessities, but they are far more stable on a day to day basis than those in the Southeast. Western tailwaters don't always have as profound an effect upon water temperature as the Southeastern counterparts as well. Some western tailwaters like the Bighorn below Yellow Tail dam are quite deep with cold flows, but others aren't as deep, and water temperatures are closer to natural. The lower Madison River below Ennis Lake fits this profile. However, Western tailwaters benefit significantly from their more stable flows. Snowmelt will still increase flows, but much of that water is held back and released over the dry season so water levels may not get as low as they would under the natural order.
A spring creek is often about the same size as a freestone river or stream but has some of the more better qualities of a tailwater. A spring creek's source of water is from a large spring or series of smaller ones. This is water that comes from deep in the earth. While this water clearly originated as rain or snow, it may have spent weeks, months, or possibly even years deep in the earth before it boiled to the surface. As a result, the water in a spring creek will have pretty constant temperatures, coming out of the ground, usually in the mid 50's year round. The water temperature can warm up slowly as the water flows away from the source, but many spring creeks will also have many small, discreet springs in the stream bed that keep the water cool. While this means water stays cold in the hot summer months is also means spring creeks are quite warm in the winter compared to freestone waters.
Spring creeks often have good numbers of large trout due to constant flows and water temperatures which allow for an incredibly dense base of aquatic insects.
The steady source of water from springs also means that spring creeks have extremely reliable flows. They may rise suddenly from runoff after a heavy rainstorm, but they rarely get low except in times of exceptional drought.
Another benefit of the spring is that the water is full of dissolved minerals which are the building blocks of life. Thick weed beds in these streams give them a distinctive look compared to freestone streams that have sparse plant life in them. Rich fertility and ideal water temperatures provide for dense populations of aquatic insects and trout.