North St. Vrain: Button Rock to Lyons
Button Rock (North St. Vrain) — Fishing Report for April 26, 2026
Quick Stats
Flow: N/A (no gauge data) | Trend: Unknown | Fishability: Good | Weather: Rain and snow showers transitioning to thunderstorms likely, high 51°F
The Bite
If there's a day to make the drive to Button Rock this spring, today might be it. An overcast, rainy Sunday afternoon is the exact recipe for a heavy Blue-Winged Olive hatch on this tailwater — the kind of afternoon where every decent lie in the canyon holds a rising brown. Expect the surface to come alive somewhere between 1 and 4 PM as cloud cover thickens and temps hover in the low 50s. The thunderstorm risk later in the day is worth watching, but the window before any lightning arrives should be prime.
No gauge data is available today, so exact flows are unknown — but the dam at Ralph Price Reservoir buffers the worst of spring variability, and with snowpack running well below normal across the basin, a major runoff pulse is unlikely. Flows should be sitting comfortably within the fishable range, probably on the lower end of the seasonal norm. Expect clear to lightly off-color water, which rewards careful wading and a stealthy approach.
Morning hours before the hatch window are worth fishing too. Scuds and aquatic worms are reliable all-day producers in the tailwater substrate, and early caddis scouts are beginning to show — the Mother's Day hatch is still building toward its May peak, but a stray olive Elk Hair Caddis in the late afternoon could draw a look.
What to Fish
- Sparkle Dun BWO #18-20 — Point fly during the afternoon hatch; let it ride the seam drag-free through the pools and tailouts
- Barr Emerger BWO #18-20 — Trail 12–18" behind the Sparkle Dun as a dropper on 6X; fish often key on emergers just below the film before committing to duns
- RS2 (olive) #20-22 — Excellent pre-hatch nymph; dead-drift through pocket water on 5X fluorocarbon
- Pheasant Tail #16-18 — Morning anchor nymph in faster boulder runs; pair with a WD-40 dropper 12" above
- San Juan Worm (wine) #12 — All-day producer near the bottom; especially useful in deeper holes before the hatch
- Scud (orange/pink) #16 — Fish tight to the bottom along any visible weed structure
Tactics & Rigging
For the afternoon hatch, go light and precise. A 9-foot 5X leader with a 12–18" 6X fluorocarbon tippet to your dropper is a solid starting setup — rig the Sparkle Dun as your point fly with the Barr Emerger trailing behind it. Approach rising fish from downstream, stay low, and make your first cast count. These browns in clear tailwater have seen pressure and will spook on a sloppy presentation. Work individual risers rather than blind-casting through the pool.
In the morning, euro nymphing techniques shine in the faster boulder runs. A Pat's Rubber Legs or weighted San Juan Worm on point with a Pheasant Tail or RS2 trailing 12" above covers the water column efficiently. Keep contact with your flies through every seam — the takes in pocket water are subtle. If the water is clear enough to spot fish in the shallower runs between boulders, a single RS2 on 5X fluorocarbon drifted past a visible brown is about as satisfying as it gets.
Access & Logistics
Button Rock Preserve is managed by the City of Longmont — no dogs, no swimming, and seasonal access hours apply. The canyon trail can be slick after rain, so traction is worth thinking about today given the wet forecast. Parking at the trailhead fills on weekends; an early arrival helps. With thunderstorms possible later in the afternoon, keep an eye on the sky and be ready to exit the canyon if lightning develops.
Stop by St. Vrain Angler for flies, local intel, and to support the shops that keep this fishery healthy.
Looking Ahead
The wet pattern holds through Tuesday, which is good news for BWO activity but worth monitoring for any flow bumps out of the dam. If you can't make it today, Monday and Tuesday look similarly overcast — the hatch window should remain reliable through midweek.