Freestone · Mixed · Montana, US

Boulder River

Fast whitewater cascading through a boulder-strewn canyon on the upper Boulder River in Sweet Grass County, Montana, flanked by conifer forest

The Boulder is a pretty little freestone off the Absaroka–Beartooth front, the kind of river you'd happily fish even if it were empty — and it isn't.

Tomorrow · About as good as it gets
Pheasant Tail
Forecast · Tomorrow
About as good as it gets
On the feed top to bottom — start on the rises with a dry, drop a nymph if they stay down.
75% projected
Today's fly
Pheasant Tail
Pheasant Tail
Conditions on the water · Tomorrow
Live gauge
Level
Steady
0.35 m
Water temp12.5°C
ClarityClear
Weather17°C
WindN 17 km/h
Pressure1009 hPa
Rain · recent0.0 mm
Rain · ahead0.1 mm

The nearest gauge doesn't record temperatures, so we've estimated it — defer to the river keeper's advice.

Water temperature for Brown Trout
Ideal
13°C est.ideal 10–16°C
0°14°28°
How to fish it · for Brown Trout
When
Nymphing can work through most of the day.
Where
Cover mixed depths.
Method
Start with tight-line nymphs and adjust if fish rise or drift higher.
Kit
9 ft #4 rod, floating line, 12 ft tapered leader to 4–5 lb fluoro tippet.
Why this works
Excellent — water temperature is right today, though insect activity could be better.
Likely species
Mayflies
Caddisflies
terrestrials
Hatch timeline · tomorrowPeak at 12pm

12pm(now) — Mahogany Dun

Stage: On emergersApproach: Emerger or dryActivity: 95%
Also:Reed Smut62%
Peak emergence windowDegree-days at peak8 food groups active

Peak activity at 12pm — Mahogany Dun (Mahogany Dun, Peak emergence window)

Sub-surface · on the nymphunder the surface

Hatch predictions

Top nymph shown — see all 2 guilds with Pro.

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Beyond the hatch · worth watchingambient prey(1)

Hatch predictions

Top prey shown — see all 2 with Pro.

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Evidence
Records-basedModerate confidence

Based on species-occurrence records, not local invertebrate surveys.

Through the year
0–3 scale · September highlighted
J
F
M
A
M
J
J
A
S
O
N
D
Trout seasonSeason
2
1
1
2
2
2
2
2
2
2
Brown DunHatch
2
3
3
3
3
3
3
2
Western Green DrakeHatch
2
2
Flav (Small Western Green Drake)Hatch
2
3
2
Mahogany DunHatch
2
2

Numbers are intensity 0 (none) to 3 (peak) — a guide, not a guarantee.

Ranked for today
Brown Trout fly box
SD
Sex Dungeon
2-6 · Pattern
Proven pattern for this period
Permits & access
Permit required — see local rules.
  • Montana licence
  • Yellowstone/westslope cutthroat are native here — handle and release them gently.
What's coming
Plan ahead
5-day outlook
Other water nearby · 5
Paradise Valley Spring Creeks
30 km
Rainbow trout · Brown trout
Shields River
46 km
Brown trout · Rainbow trout
Yellowstone River
50 km
Brown trout · Rainbow trout · Cutthroat trout
Stillwater River
60 km
81
Rainbow trout · Brown trout · Cutthroat trout
Gallatin River
88 km
Rainbow trout · Brown trout
Gallery · 2
  1. Fast whitewater cascading through a boulder-strewn canyon on the upper Boulder River in Sweet Grass County, Montana, flanked by conifer forest
    Whitewater on the upper Boulder River, Sweet Grass County, Montana
  2. Terrain map of the venue
    Terrain map
About this water

The Boulder is a pretty little freestone off the Absaroka–Beartooth front, the kind of river you'd happily fish even if it were empty — and it isn't. Rainbows, browns, brook trout, and cutthroat all live here. Once the snowmelt clears off in summer it's classic pocket-water dry-fly fishing, an attractor bouncing through the pockets and a fish meeting it more often than you'd guess.

  • Mixed
Water quality · US Clean Water Act
  • Aquatic life: Impaired
What this classification means

EPA ATTAINS assessment unit 'Boulder River' (MT43B004_132), 2021 Integrated Report. Aquatic Life use: Not Supporting. Listed impairment causes: Nutrients, Habitat Alterations, Metals Other Than Mercury. This is a Clean Water Act assessment-unit classification (the US analogue of WFD), not a live reading.

Status is for the Clean Water Act assessment unit covering this reach.

EPA ATTAINS (Clean Water Act) · MT43B004_132

The full read · show the working · for Brown Trout · confidence 90%
How the 86 is built — score × weight = contribution
Temperature100 × 28%28.0
Flow80 × 22%17.6
Clarity95 × 18%17.1
Feeding Time80 × 13%10.4
Pressure80 × 7%5.6
Insect activity57 × 12%6.8
Conditions total= 86
Can you trust it?
Water temperatureair-to-water estimateestimated
Level / flowon-river gaugeobserved
Prey activitymodelled from regional profilemodelled
Directions
Apple MapsGoogle Maps
Seasons & zones
  • Trout Third Saturday of May → Last day of February
Related guides
Euro Nymphingsubsurface methods
Booking & contacts