The Starting Lineup – 3 Flies You MUST Have for September
September is one of the best months of the year to fly fish in Colorado. Cooler nights bring water temperatures back into the ideal range, the rivers begin to quiet down, and trout feed with renewed urgency as fall approaches.
But if there is one thing that defines September fishing, it is blue-winged olives.
These small mayflies, commonly called BWOs or Baetis, become one of the most important food sources in our rivers this month. On cloudy days, the hatch can blanket the water. Even under bright skies, trout often key in on Baetis nymphs and emergers for hours.
Our Colorado River Outfitters guides have been seeing the strongest activity between 10:00 a.m. and 3:00 p.m. That window can produce everything from subtle subsurface feeding to pods of trout rising steadily in riffles, seams, and soft edges.
The challenge is that September trout have seen plenty of flies by this point in the season. The naturals are small, the fish can be selective, and the difference between a good pattern and the right pattern becomes obvious very quickly.
Rather than filling your box with dozens of options, here is the exact starting lineup the CRO guide team has been leaning on during this month’s BWO hatches:
● Day Saver Baetis — Size 22
● Sexi Top May BWO — Size 22
● Sparkle Dun BWO — Size 22-24
Together, these three flies imitate the complete BWO life cycle: the Day Saver Baetis represents the nymph, the Sexi Top May matches the emerger, and the Sparkle Dun imitates the adult. If they are in your box, you will be prepared to follow the hatch from its first subsurface activity through the height of the dry-fly fishing.
1. Day Saver Baetis Size 22
If we had to choose one fly to start with during a September BWO hatch, it would be the Day Saver Baetis.
Designed by CRO co-owner and Montana Fly Company signature fly designer Brian Hilbert, this pattern was built for the technical Baetis fishing we encounter across Colorado. Its slim profile, subtle segmentation, and small size closely match the natural nymphs trout see drifting through the water column.
That accuracy matters in September.
By late summer and early fall, trout have been pressured for months. They are accustomed to seeing artificial flies, and they often refuse patterns that are too large, too bulky, or too flashy. A size 22 Day Saver Baetis gives them a clean, realistic imitation without unnecessary material.
Before the hatch begins, BWO nymphs become increasingly active beneath the surface. They leave the rocks and vegetation where they live and begin drifting toward the areas where they will emerge. Trout recognize that movement and slide into feeding lanes to intercept them.
We fish the Day Saver Baetis throughout the entire 10:00 a.m. to 3:00 p.m. hatch window, but it is especially effective before rising fish become visible. It is an excellent trailer behind a slightly larger or heavier nymph and is equally productive under an indicator or a dry-dropper.
Wherever you fish it, keep the presentation natural. A drag-free drift and the correct depth are critical.
Why we love it:
● Precise Baetis nymph profile
● Slim enough for selective September trout
● Excellent before and during the hatch
● Ideal trailer fly in a two-fly nymph rig
● Proven on the Colorado, Roaring Fork, Eagle, and Fryingpan Rivers
Pro Tip from Brian Hilbert
“When the naturals are this small, less is more. Fish the Day Saver Baetis on light tippet, keep your rig clean, and focus on getting a drag-free drift at the fish’s level. If trout are feeding but not rising yet, this is the first fly I want on my rig.”
2. Sexi Top May BWO Size 22
The Sexi Top May BWO is built for the most exciting stage of the hatch: the emergence.
As Baetis nymphs rise toward the surface, they become vulnerable. Some struggle in the film, some fail to emerge cleanly, and others drift for several feet before escaping their nymphal shuck. Trout quickly learn that these insects are easier to capture than fully formed adults.
That is exactly what the Sexi Top May represents.
Also designed by Brian Hilbert and produced by Montana Fly Company, the pattern creates the impression of a mayfly transitioning into adulthood. Its low-riding profile and visible wing give trout the vulnerable silhouette they expect while allowing anglers to fish this fly in a variety of ways.
During a strong September hatch, watch the trout carefully. A fish that appears to be rising may actually be taking emergers just beneath the surface. You might see its back, dorsal fin, or tail without hearing the distinct sound of a true surface eat.
That is the moment to fish the Sexi Top May BWO.
Our guides have had tremendous success fishing it just below the surface film, especially as the trailing fly in a dry-dropper setup. A slightly larger, more visible dry can serve as the lead fly, with the size 22 Sexi Top May following behind it.
This pattern is especially effective in soft seams, riffle edges, foam lines, and slower water where emerging BWOs collect.
Why we love it:
● Excellent BWO emerger profile
● Effective on selective rising trout
● Can be fished below an indicator or a dry fly
● Perfect for suspending 6”-8” below your bwo dry fly
Pro Tip from Matt Fletcher
“Pay attention to how the fish are feeding, not just whether they are rising. If you see backs and tails but very few noses, the trout are probably eating emergers in or just below the film. That’s when the Sexi Top May can completely change your day.”
3.Matthew’s Sparkle Dun Size 22
The Sparkle Dun completes our September BWO lineup by imitating the adult mayfly.
Once Baetis duns successfully break through the surface film, they ride on top of the water while their wings dry. During a strong hatch, trout begin looking up and feeding rhythmically on these newly emerged adults.
That is when the Sparkle Dun shines.
Its slender body and upright wing create the unmistakable silhouette of an adult mayfly, while the trailing shuck adds a subtle suggestion of a recently completed emergence. Unlike a heavily hackled dry fly that sits high above the water, the Sparkle Dun rests low in the surface, exactly where a vulnerable BWO dun should be.
That low-riding profile is especially important when September trout become selective.
Our guides turn to the Sparkle Dun when trout are showing confident, deliberate surface rises and clearly eating adult mayflies. It can be fished by itself for the cleanest possible presentation or used as the lead fly in a dry-dropper setup with a size 22 Sexi Top May BWO trailing behind it.
That pairing covers the final two stages of the hatch at the same time: the Sparkle Dun represents the adult, while the Sexi Top May imitates an emerger trapped in the film, or just under the surface.
Focus on feeding lanes, foam lines, riffle tails, soft seams, and the edges of faster current. Watch for individual trout rising in a consistent rhythm, then place the fly slightly upstream and allow it to drift naturally into the fish’s window.
Why it belongs in your September box:
● Accurately imitates the adult BWO
● Rides low in the surface like a natural dun
● Trailing shuck adds realism and vulnerability
● Excellent for selective rising trout
● Pairs perfectly with the Sexi Top May in a two-fly rig
Pro Tip from Tim McAdow
“When trout are rising steadily, resist the urge to cast immediately. Watch one fish long enough to learn its rhythm and determine where its feeding lane begins. Then lead it with the Sparkle Dun and let the fly come to the fish. The first good cast is usually your best opportunity.”
How to Fish the September BWO Hatch
The most consistent hatch window our guides are seeing is 10:00 a.m. to 3:00 p.m., but the exact timing changes with the weather.
Cloudy, cool days often produce the strongest and longest BWO activity. Under overcast skies, adults are slower to leave the water, giving trout more time to feed on nymphs, emergers, and duns. A hatch may build gradually and continue for several hours.
Bright days can still fish extremely well, but the most concentrated surface activity may happen in a shorter window. Focus on shaded banks, riffles, foam lines, and broken water where trout feel protected.
A simple life-cycle progression will keep you prepared:
1. Nymph stage: Before the hatch, fish a size 22 Day Saver Baetis below the surface.
2. Emerger stage: As the hatch builds and trout begin feeding higher in the water column, switch to a size 22 Sexi Top May BWO.
3. Adult stage: When trout begin taking fully emerged duns from the surface, fish a size 22 Sparkle Dun.
4. During mixed feeding: Pair the Sparkle Dun with the Sexi Top May to cover adults and emergers at the same time.
5. If surface activity fades: Return to the Day Saver Baetis. Trout may still be eating BWO nymphs below the surface.
Light tippet, accurate casts, and drag-free presentations are essential. September trout are rarely impressed by a fly that moves unnaturally, no matter how closely it matches the insect.
September Is Prime Time in Colorado
If you have been waiting for the right time to plan a Colorado fly fishing trip, September is difficult to beat.
This month brings:
● Cooler water temperatures
● Consistent blue-winged olive hatches
● Fewer crowds after the summer rush
● Active trout throughout the middle of the day
● Beautiful fall colors beginning across the valleys
● Excellent opportunities for both nymph and dry-fly fishing
Our guides are currently finding productive BWO fishing on the Upper Colorado, Roaring Fork, Eagle, and Fryingpan Rivers. The best river and section can change from day to day, so every CRO trip is planned around current flows, water temperatures, hatch activity, and where the fish are feeding best.
Fish With Colorado River Outfitters
Reading about a September BWO hatch is one thing. Being on the river while trout rise all around you is another.
At Colorado River Outfitters, our guides spend every day watching these hatches develop in real time. We know which stretches are producing, when the feeding window is beginning, and how to adjust as trout move from nymphs to emergers and adults.
Whether you are a first-time fly angler or an experienced fisherman who wants to sharpen your technical dry-fly skills, we provide everything needed for a memorable day on the water:
● Experienced, professional guides
● Premium rods, reels, waders, and boots
● Custom and guide-selected flies
● Local knowledge built through hundreds of annual trips
● Instruction tailored to your experience level
September availability is limited, and this BWO window will not last forever.
We would love to show you why fall is one of our favorite times to fish Colorado.
Final Thoughts
You do not need an overflowing fly box to succeed during September’s blue-winged olive hatches.
Start with these three proven producers:
● Day Saver Baetis (#22) — The subtle, realistic nymph for selective trout.
● Sexi Top May BWO (#22) — The go-to emerger when fish move into the surface film.
● Sparkle Dun BWO (#20) — The adult imitation for trout confidently feeding on top.
Start with the Day Saver Baetis while the hatch is still developing below the surface. As trout begin feeding higher in the water column, move to the Sexi Top May BWO. When the fish are confidently taking adult mayflies from the top, finish the progression with the Sparkle Dun.
Nymph. Emerger. Adult.
With these three patterns, you can match the entire BWO life cycle without carrying an overflowing fly box. Pay attention to the hatch, adjust with the fish, and be ready between 10:00 a.m. and 3:00 p.m.
We’ll see you on the water.
—The CRO Team
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