How to Pick the Right Fly | Nerd on the Fly

"Match the hatch" is the most repeated phrase in fly fishing — and it's genuinely good advice. But it's not the whole story. Here's a practical system for choosing flies that works even when you don't know exactly what the fish are eating.

The Three Questions

Before reaching into your fly box, answer these three questions:

Are fish rising? If yes, they're eating something on or near the surface. Start with a dry fly or emerger.
What's in the air or on the water? Look for insects on the surface film, flying near the banks, or stuck to your waders. That's your target species.
What month is it? Hatches follow the calendar. A PMD hatch in January isn't happening. Know what's seasonally likely before you open your box.

If fish aren't rising and nothing is visibly hatching, default to a nymph rig. Fish eat subsurface 80–90% of the time. The dry fly moments are special precisely because they're not the default.

The Four Types of Flies

Nymph

Always — but especially when no surface activity is visible

Nymphs imitate juvenile aquatic insects living on the river bottom. Drifted below an indicator (or tight-lined if you're fancy), they're the most consistently productive fly type in Colorado. If in doubt, fish a nymph.

Pheasant Tail #16–18Hare's Ear #14–16Zebra Midge #20–22Copper John #14–18

Dry Fly

When fish are visibly rising — or during a known hatch window

Dry flies sit on the water's surface and imitate hatching or adult insects. Requires fish to be actively feeding up top. The visual eat is incredibly satisfying — but don't force it when fish aren't rising.

Parachute BWO #18–20Elk Hair Caddis #14–16Parachute Adams #14–18Sparkle Dun PMD #16–18

Streamer

Early morning, late evening, high water, or targeting large fish

Streamers imitate baitfish, crawdads, and large aquatic prey. Stripped or swung through the water. Productive in murky water when presentation matters less. The fly of choice for anglers targeting big browns.

Woolly Bugger #6–10Slumpbuster #6–8Circus Peanut #4–6

Emerger

During a hatch — especially when fish are rising but refusing your dry fly

Emergers imitate insects transitioning from nymph to adult — partially hatched, trapped in the surface film. When fish are rising but refusing your dry, they're often eating emergers just below the surface. Drop one off the back of your dry fly.

RS2 #20–22Soft Hackle Pheasant Tail #16–18CDC Emerger #18–20

Colorado's Most Important Hatches by Season

You don't need to memorize 50 fly species. In Colorado, these six account for the majority of visible hatch activity across the rivers on this site:

Fly	Season	Size
Blue-Winged Olive (BWO)	Feb–May, Sep–Nov	#18–22
Pale Morning Dun (PMD)	Jun–Aug	#16–18
Caddis	May–Sep (evenings)	#14–18
Midge	Year-round (dominant in winter)	#20–26
Trico	Jul–Sep (early morning)	#20–24
Green Drake	Jul–Aug (Colorado River)	#10–12

Fly box starting point: If you had to fish any Colorado river with just five flies, carry: a Parachute BWO #18, an Elk Hair Caddis #16, a Pheasant Tail nymph #16, a Zebra Midge #22, and a Parachute Adams #14. You'll be able to fish something productive in any month of the year.

When Fish Are Rising But Won't Take Your Fly

This is the defining frustration of dry fly fishing, and it happens to everyone. Before you change flies, check these in order:

Drag. Is your fly dragging unnaturally across the surface? Fish reject dragging flies instantly. Fix your cast before changing the fly.
Size. Go smaller. On technical Colorado tailwaters like the South Platte, dropping from a #18 to a #20 or #22 is often all it takes.
Try an emerger. Tie a RS2 or soft hackle on 18 inches of tippet off the hook bend of your dry. Now you're covering two options at once.
Change the fly. Look more carefully at what's actually hatching — size, color, body shape. You may be in the right species family but the wrong pattern.

Sizing: The Number on the Package

Fly hooks are sized on an inverse scale — the larger the number, the smaller the hook. A size 8 streamer is large. A size 24 midge is tiny. For Colorado tailwaters and technical freestone streams, you'll mostly fish in the #14–22 range. The small stuff (#18–22) is where most of the action is on pressured water.

Don't neglect tippet size. Fishing a size 20 BWO on 4X tippet is wrong — the thick tippet is visible to the fish and kills the natural drift. Match your tippet to your fly: 5X for #14–18, 6X for #18–22, 7X for #22+.

Know what's hatching before you go:

Check the Hatch Calendar →Ask the Hatch Advisor