Greybull River Hatch Chart: Key Summer Bugs and Flies

Why a Hatch Chart Matters More on the Greybull

The first time I worked the Greybull River seriously, I drove in with a fly box loaded for the Bighorn tailwater an hour east — heavy on midge larva and small mayfly emergers — and watched native Yellowstone cutthroats refuse everything for two hours while size 14 yellow sallies crawled around the boulders. Lesson learned. The Greybull is a freestone river running out of the Absaroka Mountains through the Bighorn Basin, and its bugs do not behave like the tailwaters that get all the Wyoming attention. If you do not bring the right Greybull River hatch chart in your head, you can stare at fish all afternoon without solving the puzzle.

This guide is what I keep on my phone for that drive in from Meeteetse. It is built from Greybull-specific planning notes, regional entomology references, and reports from Cody-area fishing sources, but every timing window below should be treated as a planning range rather than a fixed date. Snowpack, runoff timing, water temperature, and late-spring weather can move each emergence by a week or more. If you fish Wyoming regularly, review current Wyoming fishing regulations and creel limits before your first trip of the year — the Greybull is in Wyoming’s Area 2 regulation region, and tributary closures or special rules can change.

Clear, low summer water on the Greybull. Hatch windows can be short and condition-dependent, so use the Greybull River hatch chart as a planning edge, then verify what is actually on the water.

The Greybull Calendar at a Glance

In a typical year, the hatch year on the Greybull breaks into five broad windows that anglers from Worland, Cody, or out-of-state planning a trip should know:

• Winter through early spring (January–March): Chironomid midges all day, occasional egg patterns on rainbows staging to spawn.
• Pre-runoff spring (April–early June): BWOs on overcast afternoons, then the Mother’s Day caddis as water warms.
• Runoff (mid-May–mid-June): Streamers, worms, and waiting. The freestone goes off-color and pushes hard.
• Post-runoff summer (mid-June–August): The marquee window — golden stones, yellow sallies, PMDs, hydropsyche caddis, and hoppers.
• Fall (September–October): Trico spinner falls into early September, fall BWOs on cool afternoons, and aggressive streamer eats before winter.

What surprises most first-timers is how condition-dependent some of these windows really are. PMDs usually build from late June into July and taper as August warms. Yellow sallies often peak from late June into July, then fade as summer flows drop and daily temperatures rise. If you plan a Greybull trip the way you’d plan a Bighorn trip — show up any time, the bugs are always there — you’ll catch fish, but you’ll miss the days when the river goes from good to memorable.

Winter and Pre-Runoff: Midges and the First BWOs

From January through March the Greybull is fishable on warmer days, but the hatch chart is short and simple. Chironomid midges run year-round on this river, with peak surface activity in morning and evening windows during cooler months. Fish size 18–24 zebra midges, RS-2s, and small Griffith’s gnats. Add a small egg pattern (size 14–18, in pink or peach) from March through May when rainbow trout move shallow to spawn — but never fish directly over visible redds, and respect the cutthroat fishery the Wyoming Game and Fish Department has worked hard to restore here.

The first real surface event of the year is usually the Blue-Winged Olive emergence, which often starts in early April and can continue into early June when weather and flows cooperate. These are size 16–20 Baetis, and they show up on the worst-weather afternoons — overcast, drizzly, occasional snow squall. I rig a #18 sparkle dun above an emerger pattern on a 6X tippet, and I focus on slower seams and tail-outs where cutthroats can sip without burning calories. The classic mistake here is to fish too fast. Pre-runoff cutties are still cold and methodical; if your dry is dragging at all, they’ll refuse and you’ll never know they were watching.

Mother’s Day Caddis: A Real Window, Not a Myth

From late April into early June, the Brachycentrus caddis — the famous “Mother’s Day” hatch — can move through the Greybull when water temperatures climb before runoff takes over. On a freestone like this, the hatch can be intense for ten days and then fade quickly. Time it right and you get explosive afternoon dry-fly action with size 14–16 elk hair caddis. Time it wrong and you’ll see a few bugs lifting but no committed feeding.

The trick is timing it against runoff. If water temperatures climb into the high 40s before runoff blows the river out, you’ll see committed surface feeding for a week or two. If a warm spell pushes runoff early, the caddis still hatches but the fish are dealing with off-color water and you’ll fish caddis pupae and soft hackles on swing instead of dries. Either way, a partridge-and-orange soft hackle on a 5X dropper is the highest-percentage caddis fly on this river the entire spring.

Post-Runoff: The Stonefly Window You Cannot Miss

Once the river drops back into clarity, often sometime in mid- to late June, the Greybull’s headline window opens. Golden stoneflies (Hesperoperla) and yellow sallies (Isoperla) usually overlap from late June through July, with some year-to-year movement depending on runoff and water temperature. Cutthroats remember last year’s stones. They eat them with a complete lack of caution that you simply don’t see on tailwater rainbows or wild browns.

Run a dry-dropper rig: a #10–12 yellow Stimulator or chubby on top, a #14 yellow sally or rubber-legs nymph on 24 inches of 4X below. Bang the banks. Cast tight to undercut grass edges and rock shelves where the nymphs crawl out to molt. If you’re new to the river, this is the easiest period to find fish — the cutties hold in obvious lies and they’re looking up.

Cutthroats in clear summer water eat stoneflies and PMDs with conviction — match what’s emerging, not what worked last week. The Hatch Charts in the app are worth pulling up before you leave town: emergence windows for golden stone and yellow sally on the Greybull are tight enough that a one-week shift in spring weather can move the peak noticeably. A quick check beats showing up with the wrong size box.

PMDs (Pale Morning Duns) layer in over the top of the stone window. On the Greybull, PMDs often build from mid- to late June through July, with the strongest window commonly falling from late June into mid-July. These are size 14–18 Ephemerella, and they emerge through afternoons in slower runs and tail-outs — exactly the water the bigger cutthroats hold in. A #16 PMD parachute or sparkle dun, with an emerger trailing 12 inches behind on 5X, will outfish any stonefly imitation during the actual PMD emergence. The mistake is staying on the bank-banging stonefly program when the PMD switch flips around 1 p.m. Swap rigs, swap water — tail-outs of long pools, not the choppy banks.

Summer Caddis and Terrestrials

Through July and August, hydropsyche and brachycentrus caddis run heavy in the evenings. Tan and olive elk hair caddis in sizes 14–18 cover the surface event. During the day, a hydropsyche pupa on a tight-line drift through pocket water is a quiet weapon — most anglers ignore it because the dry hatch isn’t on yet, but the fish are eating drifting pupae from late morning through mid-afternoon.

Then comes the terrestrial program. From July into September, hoppers and ants drive a real percentage of opportunistic surface feeding when bankside grass is active and flows are low enough to concentrate fish near the edges. Greybull grasshoppers run a touch smaller than the Colorado prairie hoppers — tie on a size 10–12 Morrish hopper or chubby Chernobyl, often with a small beadhead pheasant tail dropper. Flying ants in size 12–16 become significant in late summer; when you see them on the water in numbers, the trout will key in fast and a hopper will get refused. Read what’s actually drifting before you commit to a pattern.

This is also the period when Greybull access points matter most. Low summer flows expose more wadeable water and reveal staging lies that high water hides. If you’ve fished the river only in June, the August Greybull looks like a different fishery — and the bugs look different too.

Fall: Tricos, BWOs, and Streamer Aggression

Fall on the Greybull. Cool nights drop water temps back into the BWO sweet spot and pre-winter trout feed hard on streamers.

The last act of the Greybull hatch year starts in August. Tricos (Tricorythodes) appear in late summer and run through September — size 20–24 black-bodied mayflies with dawn-to-early-morning spinner falls. Trico fishing here is a study in patience: the river you have to yourself in the morning fog, fish sipping in the back-eddies, and 6X to a size 22 spinner. It’s not high-volume fishing, but it is Greybull fishing — quiet, technical, the kind of session that makes the drive in worth it.

The BWO often returns in September and can remain relevant into October, creating the second window for size 16–20 Baetis. Fall BWOs can be more consistent than spring BWOs on this river when cooler water keeps temperatures in the sweet spot and fall fish are bulking up before winter. Overcast afternoons are best, and emergers below the surface film outproduce true duns most days.

Finally, the streamer game gets real. In fall, cutthroats and the resident brown and rainbow trout feed heavily before winter — strip articulated leeches and small sculpin patterns through deeper runs at dawn and dusk. Drop fly size to 4–8, weighted, and work the water tight to undercut banks. This is when you have a real shot at the biggest fish in the river. If the Bighorn River tailwater is your “go big” backup plan when the Greybull goes off, the fall streamer window on the freestone is when you stay put.

Building the Right Greybull Box

If you’ve read this far, the box assembles itself. Cover the hatches and you cover the river. A focused Greybull box looks like this:

• Mayflies: BWO parachute and emerger 16–20, PMD sparkle dun and emerger 14–18, trico spinner 20–22
• Caddis: Elk hair caddis 14–18 in tan and olive, partridge-and-orange soft hackle 14–16, hydropsyche pupa 14–16
• Stoneflies: Yellow Stimulator 10–14, rubber-legs in olive/brown 8–10, yellow sally 14–16
• Terrestrials: Morrish hopper 10–12, foam ant 14–16, beetle 14–16
• Subsurface staples: Zebra midge 18–24, pheasant tail 14–18, San Juan worm 10–14 (huge after high water), small egg patterns 14–18 in spring
• Streamers (fall priority): Articulated leech, sculpin patterns, Woolly Bugger 4–8

That’s it — five rows, no padding. The Greybull rewards a small, river-specific box over a big general-purpose one.

Bringing It All Together

The Greybull is a research-it-first river. Native cutthroats, narrow hatch windows, and remote water make it a place where preparation pays back tenfold. Before you commit to the drive, cross-reference the calendar above against actual conditions: water temperature, water clarity, recent weather, nearby gaged rivers, and what’s been hatching. The app currently has no live Greybull discharge reading. Nearby gauges provide regional context, not a measurement of the water at your planned crossing. The Hatch Charts and River Conditions inside the FlyGuysNLies app help line up likely bug emergence windows with current basin conditions — pull both up the night before so you tie on the right top fly the first time. While you’re planning, double-check Wyoming’s native cutthroat conservation regulations and creel limits so you can fish the river right.

The fish here are not gullible just because they’re remote. They have seen the same bugs every year of their lives, and they will refuse a sloppy match faster than tailwater fish ever will. But get the season right, get the bug right, and the Greybull will hand you a kind of fishing you cannot find anywhere else in the Bighorn Basin. That’s what a good hatch chart is really for — turning a hopeful drive into a planned one.

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