Choosing the right fly comes down to three things: where fish are feeding in the water column, what insects are currently active, and what the conditions look like around you. You don’t need to memorize hundreds of patterns. You need a system for reading the water and narrowing your options, and that system is simpler than most anglers make it sound.
Start With the Water Column
Every fly falls into one of three broad categories based on where it fishes. Dry flies float on the surface and imitate adult insects sitting on or stuck in the film. Nymphs sink below the surface and mimic the immature, underwater stage of aquatic insects. Streamers are larger, weighted flies that imitate baitfish, leeches, or crayfish and can be fished at nearly any depth. Your first decision isn’t which specific pattern to tie on. It’s which category to fish.
If you see fish rising, visibly breaking the surface to eat, start with a dry fly. If the water looks quiet with no surface activity, fish a nymph. This isn’t a guess. Trout eat subsurface roughly 80 to 90 percent of the time, so nymphs are the default. Streamers come into play when you’re covering a lot of water, targeting bigger fish, or fishing in off-color conditions where subtlety doesn’t matter.
Read What’s Hatching
The phrase “match the hatch” is the foundation of fly selection. It means identifying the insects currently emerging from the water and choosing a fly that resembles them in size, shape, and color. Three groups of aquatic insects make up the bulk of a trout’s diet: mayflies, caddisflies, and stoneflies. Each one looks and behaves differently, and you can tell them apart without a magnifying glass.
Mayflies have slender bodies, upright vertical wings, and two or three delicate tails. Their flight is graceful and rhythmic, tracing a measured up-and-down wave over the water. When mayflies are emerging, trout tend to rise with slow, deliberate sips because the insects float on the surface for a moment while their wings unfold, giving fish time to eat them casually.
Caddisflies look completely different. Adults have four wings that fold back along their bodies like a tent. Their flight is erratic and chaotic, spiraling and bouncing unpredictably over the water. When caddis hatch, they can fly almost immediately after breaking through the surface film, so trout feeding on them make splashy, aggressive rises, sometimes clearing the water entirely. If you see that kind of surface activity, reach for an elk-hair caddis or similar tent-wing pattern.
Stoneflies are the heaviest of the three. Their two pairs of long wings beat slowly and deliberately, pushing them in a straight line through the air. Large stoneflies are sometimes big enough that you’ll feel one crash into your arm before you see it. They typically emerge along the shoreline rather than in open water, so fish feeding on stoneflies tend to hold near the banks. Foam-bodied patterns with rubber legs are the go-to imitation.
Here’s a useful trick: watch the birds. If swallows or other insect-eating birds are flying in straight lines from the bank and back to their perch, they’re picking off mayflies. If their flight path looks erratic and darting, they’re chasing caddis.
Use the Season as a Starting Point
You won’t always arrive at the river during an obvious hatch. When nothing is visibly emerging, the time of year narrows your choices considerably.
Midges are the one insect group that hatches year-round, emerging in the afternoon across every month. In the dead of winter, tiny midge patterns (sizes 18 to 22) are often the only game in town. Blue-winged olives, a common mayfly, overlap with midges in late winter and early spring, typically appearing on warm afternoons from February through April. They also make a second appearance in October and November. If you’re fishing a cold, overcast afternoon in early spring, a small olive-colored mayfly pattern is a strong bet.
Late spring and early summer bring the largest variety of hatches. By June, terrestrial insects enter the picture. These are land-based bugs like ants, beetles, inchworms, and grasshoppers that get blown or knocked into the water. Consistent warm, sunny weather is the key trigger. Cold nights in late spring keep terrestrials hiding, and wet, cloudy summers can delay their season significantly. Start with ants, beetles, and inchworms early in the season, then switch to grasshopper patterns from late June onward once the hoppers have grown large enough to interest fish. Two or three consecutive days of sunny weather is a reliable signal that terrestrial fishing will be good.
During the heat of summer, the best terrestrial fishing often shifts to dawn and dusk, when water temperatures cool and trout feel safer feeding.
Water Temperature Matters
Trout feed most actively in water between 45 and 65 degrees Fahrenheit. Below 45, their metabolism slows and they conserve energy, making them reluctant to chase food. This is when small, slow-drifting nymphs and midges are most effective because fish won’t move far for a meal. Above 65 degrees, trout become stressed. At 67 degrees and above, fishing for trout is discouraged entirely because the exertion of being caught can kill them even if you release them quickly.
In that prime 45-to-65 range, fish are willing to eat across the water column. The warmer end of that range tends to produce the best dry-fly fishing because insect hatches are more prolific and trout are energetic enough to rise to the surface.
Size Before Pattern
Getting the size right matters more than picking the perfect pattern. A reasonably close imitation in the right size will outfish an exact replica that’s too big or too small. Fly sizes use a numbering system that runs counterintuitively: the larger the number, the smaller the fly. A size 18 is tiny, a size 12 is moderate, and a size 6 is large. Sizes smaller than 2 come in even numbers only, so you’ll jump from 12 to 14 to 16.
If you can see the natural insects on the water, catch one in your hand or scoop one off the surface and compare it to what’s in your fly box. When in doubt, go one size smaller than you think you need. Fish see these insects constantly and are more likely to refuse something that looks oversized than undersized. Very large hook sizes, labeled with “ought” numbers like 1/0 or 2/0, are reserved for streamers, bass flies, and saltwater patterns.
Adjust for Water Clarity and Light
The color of your fly should respond to conditions, not just the insect you’re imitating. In clear water on sunny days, you have the most flexibility. Red, orange, white, and black all work well near the surface because there’s plenty of light for fish to see contrast and color. Black is especially effective because it creates a sharp silhouette against a bright sky.
As conditions get darker, whether from cloud cover, depth, or murky water, colors drop out in a predictable order. Red disappears first, looking gray to fish from as little as 17 feet down in clear water and vanishing almost immediately in stained or dirty water. Yellow and green go next. In low-light situations or deep water, blue, purple, and black are the colors that remain visible longest. If you’re fishing at dusk or running streamers deep, darker flies are your best option.
Stained water that looks like tea or coffee is a special case. Golden, red, and orange tones stand out well in those conditions as long as there’s some daylight. In water thick with algae, chartreuse cuts through better than anything else because green light is the only wavelength that penetrates an algae-heavy water column.
When Nothing Works, Go General
Not every situation calls for a precise imitation. Attractor patterns are flies that don’t closely resemble any specific insect but provoke strikes through a combination of flash, movement, color, and profile. They work because trout are opportunistic. A fly that looks like “something alive and edible” is often good enough, especially in faster water where fish have less time to inspect their food.
A Parachute Adams in sizes 12 to 16 is one of the most versatile dry flies you can carry. It floats well, is easy to see, and passes for both mayfly adults and spent spinners. A Woolly Bugger, dead-drifted through deeper water, covers nymph and streamer duties at the same time. These general-purpose patterns won’t always outfish a precise match during a heavy hatch, but they’ll catch fish on days when you can’t figure out exactly what’s going on.
The Euro-nymphing approach takes this idea even further, favoring nondescript, weighted flies over precise imitations and putting the emphasis on getting the fly to the right depth with a natural drift. It’s a reminder that presentation, getting your fly where the fish are and making it behave naturally, often matters more than the specific pattern on the end of your line.