Fall BWO Season on the South Platte — October Fly Fishing Guide

September and October on the South Platte are the best-kept secret in Colorado fly fishing — and I’m a little annoyed at myself for telling you, because the parking lots are finally empty and now look what I’m doing. The summer crowds are gone. The PMDs are done. The caddis have mostly finished. What remains — for six to eight weeks of reliable surface fishing — is the fall Blue-Winged Olive hatch, and it’s better than the spring version in almost every way. The South Platte hatch calendar shows exactly where the fall BWO window falls relative to the rest of the season.

This is the guide to fishing it correctly — so you don’t spend a perfectly good gray October morning doing what I did for years, throwing a #18 at fish that wanted a #22.

Why Fall BWOs Beat Spring

Most South Platte anglers are aware of the spring BWO hatch. Fewer pay proper attention to the fall one. Here’s why the fall is actually better:

Calmer water. Spring on the South Platte involves runoff pressure, fluctuating flows, and cold, fast water that makes flat-water dry-fly presentations difficult. In September and October, flows have settled into their fall baseline — typically lower and more stable than spring — and the flat sections at Cheesman and Deckers fish with the gin clarity and slow current that produces the best dry-fly conditions of the year.

Longer hatch windows. Spring BWO hatches on the South Platte tend to be compressed — 30 to 45 minute windows triggered by brief overcast periods. Fall hatches, particularly in September and early October, can run for 2–3 hours on the right day. The lower water temperatures and the stability of fall weather in Colorado create extended emergence windows.

Fewer anglers. The crowd that was at Cheesman in May is not there in October. A Thursday morning in mid-October will give you access to water that had 20 anglers on it in June. The fish have had months to recover from summer pressure and they’re feeding aggressively ahead of winter.

Active fish. Brown trout begin pre-spawn behavior in October. They’re more aggressive, more territorial, and more willing to commit to a surface fly. The largest browns of the year are visible and active in fall in ways they simply aren’t in the summer.

When It Happens

Season: Late August through mid-November, with the best fishing in late September and October.

Daily timing: BWOs hatch in response to overcast conditions, dropping barometric pressure, and cooler air temperatures. The fall hatch timing is less predictable than the spring version — instead of a reliable 10 AM window, fall BWOs can emerge at any point from 9 AM through 3 PM depending on cloud cover and temperature.

Best day type: Overcast, 45–60°F air temperature, ideally with a slight chance of rain. Bluebird fall days in Colorado produce minimal BWO activity. If you’re planning an October South Platte trip for BWOs, hope for gray skies.

What size BWO fly works best in the fall on the South Platte?

Start with a #22. Fall BWO duns on the South Platte run on the smaller end of the size range — #22 beats #20 on most days, and #24 can be necessary on fish that are clearly rising but refusing everything. Pair it with 7X tippet for the low, clear October water.

Key transition: When temperatures drop into the 30s overnight and air temps struggle to reach 50°F by midday (typically mid-October to early November), the hatch window compresses and the fish move from surface feeding to deeper holding. This is the end of the fall dry-fly season.

Best Sections

Cheesman Canyon — the premium fall BWO destination. The canyon walls create shade and shelter that concentrates BWO emergence even when the valley above is in partial sun. The deep, clear pools hold large trout in visible feeding lanes. October weekday mornings at Cheesman produce some of the most technical and satisfying dry-fly fishing in Colorado.

The run below the Gill Trail junction and the slot above the third canyon wall are the two most consistently productive fall beats. Both require the standard canyon approach — fish are visible and spooky, long leaders and careful approaches are non-negotiable.

Deckers — easier access and reliable fall BWO activity throughout the Gold Medal section. The flat above Trumbull Road bridge concentrates fall risers well. Less technical than Cheesman, less demanding approach, similar quality fish.

Eleven Mile Canyon — the canyon’s consistent cold water and shaded walls make it one of the best fall BWO sections in the system. The mid-canyon section (miles 2–5) produces extended morning BWO activity on overcast days that outlasts what you’ll find at Deckers. Fewer anglers than either Cheesman or Deckers in October.

The Flies

Dry Flies

Vis-A-Dun (#20, 22) — the most effective fall BWO dry fly on the South Platte. A CDC-winged pattern with a dubbed olive or gray body. The CDC wing creates the right translucency and lies flush in the surface film in a way that triggers the selective trout feeding on fall BWOs.

The key detail: fall BWO duns on the South Platte tend to run on the smaller end of the size range — #22 is more effective than #20 on most days, and occasionally #24 is necessary on fish that are clearly rising but refusing everything. Start with 22.

CDC Comparadun BWO (#20, 22) — a comparadun with a CDC wing fanned in a semicircle. Effective as an alternative when fish are refusing the Vis-A-Dun. The different wing profile occasionally triggers fish that have locked onto a specific silhouette.

Parachute BWO (#20, 22) — the visibility option for difficult light conditions. White parachute post, olive body. Use it when you need to see your fly on the surface from a distance — morning light, rippled water, or when fishing to spotted risers 40+ feet away.

Sparkle Dun BWO (#20, 22) — a Sparkle Dun tied in BWO colors with trailing Antron shuck. Effective during the transition moment when duns are still hatching from their shucks — typically the first 20 minutes of the hatch. As the hatch matures and fish key on the fully-formed adult, switch to the Vis-A-Dun or Parachute.

Subsurface

RS2 (#20, 22) — the most versatile BWO emerger on the South Platte. Gray or olive, small sized. Fish it as a dropper below your dry fly during the approach phase of the hatch, before fish have committed to the surface. Also effective on days when you see subtle rises (barely-breaking-the-surface sips) that suggest fish are eating emergers in the film rather than adults on top.

Soft Hackle BWO (#18, 20) — partridge and olive or partridge and hare’s ear. Swing it through current seams during the hatch — particularly in Eleven Mile’s riffle sections where trout intercept emergers in the current rather than rising to adults.

All of the above are available at The Fly Fishing Place — including the Parachute BWO #20, Parachute BWO #18, and Soft Hackle. Our link gets you 15% off everything.

Rigging

Standard fall BWO rig:

• 12-foot 5X leader (longer than spring — the lower, clearer fall flows demand it)
• 24 inches of 7X fluorocarbon tippet
• Single dry fly

7X is the standard, not the exception, for fall BWO fishing on the South Platte. The low, clear water of October combined with the small fly sizes (#22) require fine tippet. 6X will catch fish but refusal rates climb noticeably.

Before the hatch:

• 9-foot 5X leader, 24 inches of 6X
• RS2 or Soft Hackle BWO dropper
• Switch to dry-only as surface feeding begins

Approach and Tactics

Fall fish on the South Platte are wary in a specific way: they’ve been fed on and caught for 5–6 months. By October they have a PhD in tippet and they know what a drag looks like. They’ve seen 10,000 flies, and they were not impressed by most of them.

What this means practically:

• Position matters more than pattern. Getting your drift through the feeding lane without drag is more important than which specific BWO pattern you’ve chosen.
• Don’t false cast over rising fish. Position yourself so your false casts go away from the feeding lane. The flash of fly line overhead moves fish.
• Time your presentation. Watch the rise rhythm. Most BWO-feeding trout develop a regular cadence — rising every 8–12 seconds. Present your fly so it arrives during the active feeding window, not between rises.
• Go longer on the drift. Fall fish are slower and more deliberate than summer fish. Let the fly drift farther than you think you need to before picking up for the next cast.

October Access Notes

Cheesman Canyon access requires the Gill Trail approach. The trail is in good condition through October. After the first hard freeze, check for ice on the shaded sections of the trail in the mornings — occasional microspike conditions in late October.

Deckers road access is straightforward year-round. Parking is not an issue in October — the lots that fill by 6:30 AM in summer are rarely full before 9 AM in fall.

The Dream Stream requires checking Spinney Mountain SWA regulations — some access points have seasonal closures related to waterfowl hunting seasons in October. Verify before you go.

So pick a gray, drizzly, miserable-looking day — the kind your non-fishing friends will think you’re crazy for chasing — and go. The crowds won’t, the fish will be up, and you’ll have the best dry-fly water in Colorado mostly to yourself. That trade has never once felt like a bad one to me.

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