Spring trout window open on the Yellowstone ahead of peak runoff

What's Biting

What's Next

The Yellowstone's current reading of 1,070 cfs at Corwin Springs places it in an early-spring buildup phase. This is a snowmelt-driven system, and flows can accelerate rapidly once sustained warm temperatures settle over the surrounding mountain ranges — expect levels to climb through mid-to-late May, potentially reaching multiples of today's reading within a few weeks. Anglers planning wading trips should move sooner rather than later; once river color shifts from green to gray and current velocity picks up, wade-fishing becomes hazardous and sight-fishing quality collapses.

For the near term, if temperatures stay mild, the best trout action will concentrate on inside bends, sloughs, and moderate-depth pool tail-outs where faster water slows and fish can hold without burning energy. Early May on the upper Yellowstone typically brings midge and BWO activity in the cooler morning hours, with caddis emergences building through afternoon as air temperatures rise — Hatch Magazine notes caddis patterns have long been productive on the Yellowstone system once spring hatches kick in. Carry a range of nymph patterns including caddis pupae, soft hackles, and small stonefly imitations to cover the likely hatch spectrum. No regional guide or shop reports are available this week to refine specific fly selections further.

On the Missouri tailwater stretches — the regulated sections below the main impoundments that hold brown and rainbow trout year-round — flows are far more stable than on the free-flowing Yellowstone, making them a reliable fallback if the main river colors up. These fisheries tend to hold consistent trout populations through much of runoff season. Walleye on the broader Missouri main stem should be entering an active post-spawn feeding mode typical for mid-May; channel edges and current seams are the traditional holding areas, and jig-and-minnow or live-bait rigs along those edges are the standard spring approach.

Two near-term items worth circling: MT FWP is hosting a public meeting on proposed 2027–2028 fishing regulation changes at the Billings Public Library on May 11 at 6 p.m. — a direct opportunity for anglers fishing these waters to weigh in on 41 preliminary proposals. And the lottery for upper and West Fork Bitterroot commercial use permits closes May 18, per MT FWP, for anyone interested in guided float operations on that restricted river. Conditions can change quickly in May — check USGS gauge 06043500 before loading the truck.

Context

For Montana's Yellowstone and Missouri drainages, early May sits at a seasonal hinge between fishable spring conditions and the roiled high water of peak runoff. The Yellowstone is a snowmelt-driven system, and the timing of peak discharge varies considerably year to year depending on winter snowpack accumulation and spring temperature trajectory. In average years, the upper Yellowstone at Corwin Springs climbs steadily through April and May, reaching peak discharge — frequently in the high thousands of cfs — sometime between mid-May and mid-June before dropping back to summer baseflows. A May 10 reading of 1,070 cfs is consistent with an early-to-mid buildup phase, before the main runoff pulse has fully developed. Whether this season is running early, late, or on schedule relative to historical averages cannot be determined from the data available this week.

The PFAS story is the standout contextual development shaping the 2026 season across Montana. MT FWP, alongside state health and environmental agencies, released formal fish consumption advisories after testing detected PFAS chemicals in fish tissue from some Montana waterbodies. Flylords Mag also reported that an internal FWP document showing contamination had reportedly been suppressed since 2024 before this public disclosure. The specific affected waters are not enumerated in the available feeds, but this is a meaningful shift in the health calculus for anglers who regularly eat their catch — particularly on waters near agricultural or industrial land use. Consulting the current MT FWP advisory page directly is the only reliable way to determine whether a specific stretch is affected.

From a hatch perspective, Hatch Magazine's coverage of Yellowstone-system caddis emergences is a useful seasonal reminder: late spring marks the transition toward the region's most productive dry-fly conditions, but that window typically opens in earnest after runoff subsides and clarity returns — usually June on the free-flowing upper Yellowstone, with tailwater fisheries often coming on sooner. Anglers targeting the pre-runoff window now are fishing ahead of that prime window, which can mean less competition and respectable nymph fishing if conditions cooperate.