Riffles are the preeminant feature of coldwater streams. They are at once a food source, a shelter from predators, a hedge against oxygen depletion, and a conveyor belt that brings food to the trout. Riffles, with their broken water surface, not only hide the trout from predators, but also hides predators (such as humans) from the trout. Because of this, trout in riffles may be approached more easily, and are harder to spook. Many species of insects reproduce or grow to maturity in riffles. The constant fast current dislodges nymphs from the rocks, freeing them into the "biological drift", a term that refers to the constant downstream movement of organisms in flowing water. Riffles also oxygenate the water. In hot weather, trout may congregate in riffles, where the oxygen content of the water is highest. Riffles may be any depth, but most are between one and three feet deep. Within a riffle, trout may lie in wait behind rocks, hug the bottom, or roam about. Small pockets of deeper water in a riffle are prime feeding locations for trout, and invariably hold good fish. Riffles also hold larger prey items, like darters, sculpins, and crayfish, so large trout may move into riffles periodically to feed, especially at night.
Runs are similar to riffles, but although their current may be somewhat swift, their surface is smooth enough to allow light to penetrate. Runs are characterized by moderate current and a smooth surface. Runs may be deeper than riffles, but this depends on the size of the stream. Runs that form bends may form undercut banks, as the current erodes the underside of the streambank. Trout use runs as both holding and feeding areas. Trout do not need to expend as much energy fighting current in a run as in a riffle, so when food is abundant, trout may move out of a riffle and into a run to save energy while feeding.
Pools are one of the most obvious features of a stream. They are popular with beginners who become mystified by the trout they see lurking in pools. Pools often hold suckers as well as tout. The pool provides the two things that are generally lacking in coldwater streams: depth and still water. The deep water of a pool provides a trout with the ultimate in protection from predators. However, because current in the main pool, especially near the bottom, is almost nonexistant, food is hard to come by there. Where a riffle or run enters a pool,a featured called "the toungue of the pool" is created. This area is where all biological drift enters the pool, and is a prime location for trout to lie in wait. The entire upstream end, where the tongue is, is called the "throat". The deepest section in the middle of the pool is called the "belly" and the narrows at the bottom where the water speeds up as it exits the pool is called the "tail". The tail concentrates food, and any kind of structure located in the tail of the pool is a prime location which will hold fish.
A flat might be called a shallow pool. Flats have a still, unbroken surface, but a shallow, uniform bottom. Flats may or may not be productive, depending on bottom type. Smooth, sandy flats are almost worthless as trout habitat, except at the edges or near woody debris. Gravel flats are better, but flats that are filled with aquatic vegetation are perhaps the best. Open channels that often form between the weeds are perfect holding spots for trout, but beware: trout on flats are incredibly wary and can see the area above the water perfectly. Fishing for trout in flats is a place where presentations must be artful, tippets must be long and fine, and trouters must make every effort to conceal themselves from their quarry.
Wherever strong current flows against an earthen bank, the area beneath the water may become eroded. This creates a submerged, cavelike overhang in which trout may hold without worrying about predators. Undercut banks are also created by man, these so-called "Lunker Structures" are placed in the stream to provide additional cover for trout in areas where undercuts do not occur naturally. In all cases, these stream features will hold fish. Presenting a fly to these fish, however, can be quite a challenge. Depending on the current, it may be possible to drift a nymph beneath an undercut, but more often than not this is an exercise in futility. Creeping up on the same bank and dapping your fly over the edge works occasionally, and during a hatch, a dry fly may be drifted against the bank to elicit strikes from the trout concealed beneath the undercut.
Logs, branches, even whole trees sometimes end up in trout streams. These features block the current and provide shelter for fish. Swinging a streamer from upstream is one presentation that works in these instances. Woody debris, when combined with another feature, such as deep water or the tongue of a pool, is a trout magnet.
Whether it is tall grass or tree branches, anything hanging out over a stream is worthy of notice. These structures protect trout from their most effective predator, the fisherman. Plus, terrestrial insects, such as ants, aphids, and beetles, may drop into the stream from such. A carefully planned cast that is allowed to drift beneath overhanging vegetation is always worth a shot. Or, you can creep up on the bank and gently lower a tiny ant imitation onto the surface of the water and feed out line to let it drift beneath a tree. This crafty and highly enjoyable tactic has always been enjoyable.
Small waterfalls will occasionally be encountered on trout streams. Where the falling water hits soft bottom, a hole is scoured out that may be considerably deeper than the surrounding water. Trout love these tiny, sheltered pockets and a weighted nymph, cast above the waterfall and allowed to travel down to the bottom of the plunge pool, will take fish.
Rapids and Pocket water
Rapids are areas where the water is so swift that trout do not hold in them. Within a rapid, however, fish will maintain station in scour holes, behind rocks, and in small "pocket water" of various types. Heavily weighted nymph rigs may be employed to explore these areas.