Weekly Hatch Report: Trumbull Section — May 2026

The South Platte below Cheesman is in a transitional window right now, and understanding what that means for your success this week is critical. We’re moving out of the heavy spring runoff phase, but the water is still pushing volume through the canyon. Before you load the truck, get on the USGS gauge at the Deckers station—that number will dictate which beats fish well and which ones turn into washing machines. Higher flows compress fish into slower water and reduce visibility. Lower flows open up the mid-channel structure and allow presentations to penetrate where trout are actually holding. Either way, the tailwater is waking up to late spring conditions, and the insect activity reflects it.

What’s Hatching

May in the Trumbull section means you’re in the middle of a prolonged mayfly window. The blue-winged olives are still thick—particularly the Baetis species that dominate the South Platte ecosystem. Fish #18–#20 olive patterns, both dries and nymphs. Olive Sparkle Duns and CDC Olives work on the surface; underwater, standard BWO Pheasant Tail nymphs in #18–#20 remain essential. Don’t overlook the smaller #22 offerings—the afternoon hatches often feature smaller duns, and matching that size pays dividends.

Caddisfly activity is ramping significantly. You’re looking at Hydropsyche species (tan, tan/brown colorways) in #14–#16, and the grannom population is building toward its peak. Fish Elk Hair Caddis in tan and brown, or switch to soft-hackle patterns like the March Brown Soft Hackle if you want a suggestive imitation that works in faster water. On the nymph side, tan Caddis Pupae in #16 will work throughout the day, but focus on them during low-light periods and when you see actual activity.

Little yellow stoneflies are beginning to show. These aren’t the thick flights you’ll see in June, but they’re present enough to matter, especially in the slower margins and inside bends. A #16 Yellow Sally, either dry or nymph form, should be in your box and ready to deploy if you spot them.

Midges remain a constant. Black midges in #24–#26 are never wrong on the South Platte. Fish them as emergers or dries, particularly during the lower-light hours and in deeper pools where light penetration is limited.

What’s the best fly for Trumbull right now?

A #18–#20 olive pattern, dry or nymph. The blue-winged olives are still thick this week, so a BWO Pheasant Tail underneath or a CDC Olive on top covers most of the water. If they’re ignoring that, drop to a #24–#26 black midge.

Best Beats

Manitou Park to the Deckers Bridge: This stretch fishes well across a wider range of flow conditions. During higher water, the eddies along the north bank and the slow water immediately below the bridge create refuges where trout bunch up. The deeper runs here also allow you to work subsurface presentations effectively without constantly getting swept downstream. The takeaway: this beat produces more consistently when conditions are marginal.

The Narrows, downstream from mile marker 2: When flows are moderate to low, this section opens up nicely. The main current is defined, but there’s exceptional structure—pocket water, undercut banks, and current seams that hold pods of fish. Caddisfly nymphs and Pheasant Tail drifts through the seams are money here, especially in the afternoon. The narrower canyon also means faster air movement, so terrestrial possibilities increase as the week progresses if temps climb.

Below the Wigwam Club private water: The public access point here gives you entry to a section that sees less pressure than areas closer to the trailhead. The water settles into more defined pools with slower margins. During higher flows, this is an overlooked zone where trout are easier to locate because they’re less scattered. Olive duns and soft-hackle caddis patterns work throughout the day.

Tactics

Build your leader around 9-foot, 4X or 5X fluorocarbon, depending on clarity. At standard spring flows with some color in the water, 4X gives you better turnover and more insurance against break-offs in the current. If flows drop significantly and you can see the bottom clearly, drop to 5X. Tippet: 5X for dries; 4X for nymphs and soft hackles.

Nymph rigs: go with a two-fly setup. Lead with a heavier Pheasant Tail (#18) and tag a smaller Olive Emerger (#20) or small Yellow Sally (#16) off a 12–18 inch dropper. Weight the rig with split shot positioned 18–24 inches above the lead fly. This gets you down quickly without locking up in the faster current.

For dry-dropper work—increasingly productive as hatches intensify—use a CDC Olive or Elk Hair Caddis as your point fly and hang a soft-hackle or emerger pattern 18–24 inches below. This covers both surface feeders and subsurface risers in a single drift.

Practical Notes

Afternoon canyon winds are becoming regular now. Expect gusts that can push you around and demand tighter casting. Keep your backcast tight and consider shortening your casting stroke when the wind builds.

Access note: Parking at the Trumbull trailhead fills on weekends, and early-week traffic varies. Arrive early or plan to fish later in the day when turnover is higher.