Yellowstone caddis hatches building ahead of runoff

What's Biting

What's Next

The 771 cfs reading at USGS gauge 06043500 puts the Yellowstone in a favorable mid-spring window. Montana's characteristic spring runoff — driven by high-country snowmelt — typically accelerates through mid-to-late May, often pushing Yellowstone mainstem flows to multiples of their current level and dramatically reducing visibility. Practically speaking, the next two to three weeks represent a prime opportunity to fish the river before conditions deteriorate. Fish it now.

Caddis emergences are the story this week. Hatch Magazine, with longstanding coverage of Yellowstone hatches, notes that caddis are a foundational hatch anglers should key on during this period, building steadily through mid-May. Expect peak emergence windows in the afternoon, roughly 2–5 p.m., when water temperatures climb enough to trigger sustained adult activity. An Elk Hair Caddis or LaFontaine Sparkle Pupa in sizes 14–16 covers the dry-fly and emerger angles respectively; rigging either pattern beneath a larger attractor dry as a dropper gives you both windows at once. Nymphing productive seams and softer pockets inside bends remains the consistent all-day producer when hatches aren't fully on.

Float access planning is critical this weekend. The Grey Bear Fishing Access Site boat ramp on the Yellowstone is offline until approximately May 21 per MT FWP Fishing News — scout alternative launch and take-out points well in advance and confirm no additional closures at myfwp.mt.gov/fwpPub/allRestrictions.

If snowmelt accelerates and the Yellowstone rises sharply over the coming days, the Missouri River's tailwater sections offer a reliable refuge with more stable temperatures and visibility year-round. Post-spawn walleye on the Missouri should be actively feeding in transition zones near current edges and structural breaks — jig presentations worked along bottom contours are a proven tactic when main-stem rivers run off-color.

The waning gibbous moon correlates with feeding activity concentrated more heavily in daylight hours. Prioritize mid-morning through late afternoon windows. MT FWP Fishing News also notes a public meeting on 2027–2028 fishing regulation proposals in Billings on May 11 (6 p.m., Billings Public Library) — worth attending if you want input into how these waters are managed going forward.

Context

For early May on the Yellowstone, 771 cfs at USGS gauge 06043500 represents a moderate pre-runoff reading. The Yellowstone's major snowmelt surge typically arrives in late May through June, when the upper and mid-river can climb sharply. An early-May reading in the 700s suggests high-country snowpack has not yet entered peak melt — consistent with typical seasonal timing for this drainage. Conditions are manageable now, but a significant rise is likely on the horizon within two to four weeks.

Caddis emergences building in early-to-mid May on the Yellowstone system are entirely on schedule. Hatch Magazine's long-running coverage of Yellowstone hatches confirms that caddis define the spring period on these waters, and a mid-season window between melt events is classic timing for them to fire. There is nothing in this year's data suggesting an unusually early or late season — it reads as a relatively on-schedule spring.

The PFAS fish consumption advisory is a newer and significant development worth contextualizing. MT FWP, DEQ, and DPHHS released data showing detectable PFAS levels in fish from some Montana waterbodies — a meaningful shift in how anglers should approach keeping fish from affected reaches. Flylords Mag has additional reporting noting that an internal FWP report documenting these findings was reportedly suppressed since 2024. This is not a blanket fishing prohibition, but anglers should consult the specific advisory list before keeping any catch. This development has no historical precedent for Montana and stands apart from seasonal patterns.

Flylab (Substack)'s feature piece on the Madison River underscores that the Madison drainage — feeding ultimately into the Missouri system — remains one of Montana's signature spring trout destinations, carrying the same caddis-and-nymph rhythm that defines fishing across the state's interconnected river systems. No comparative signal in this cycle's intel suggests dramatically anomalous conditions; the main story is simply that the pre-runoff window is open and the clock is ticking.