Tailwater · Mixed · Idaho, US

Big Lost River

A river meanders through a wide agricultural valley with a viewing bench and interpretive sign in the foreground and snow-covered mountains behind
Today · About as good as it gets
Czech Nymph
Forecast · Today
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.
90% projected
Today's fly
Czech Nymph
Czech Nymph
Conditions on the water · Today
Live gauge
Level
Falling
0.53 m
Dropping after lift
Water temp10.1°C
ClarityClearing
Weather10°C
WindN 4 km/h
Pressure1026 hPa
Rain · recent0.0 mm
Rain · ahead0.0 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
10°C est.ideal 10–16°C
0°14°28°

We have not characterised this dam's release, so this is a general cold-release figure and could be several degrees out in either direction. Treat it as a hint, not a reading, and check locally before you fish.

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 time of day could be better.
Likely species
Mayflies
Chironomids
Gammarids
Hatch timeline · todayPeak at 11 Uhr

12 Uhr(now) — Western Blue-Winged Olive

Stage: On emergersApproach: Emerger or dryActivity: 83%
Also:Black Midge63%
Peak emergence window6 food groups active

Peak activity at 11 Uhr — Western Blue-Winged Olive (Western Blue-Winged Olive, Peak emergence window)

Sub-surface · on the nymphunder the surface

Hatch predictions

Supporters see the full sub-surface read.

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Beyond the hatch · worth watchingambient prey(1)

Hatch predictions

Top prey shown — see all 3 as a supporter.

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Evidence
Records-basedModerate confidence

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

Through the year
0–3 scale · October highlighted
J
F
M
A
M
J
J
A
S
O
N
D
Western Blue-Winged OliveHatch
2
3
3
3
3
3
3
3
2
Pale Morning DunHatch
2
3
3
2
TricoHatch
2
3
2
Black MidgeHatch
2
3
3
3
3
3
3
3
3
3
3
2

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

Ranked for today
Fly box · Brown Trout
Permits & access
Permit required — see local rules.
  • Idaho licence
  • Flow is irrigation-controlled and can change overnight
  • Special-regulation reaches apply — check Idaho Fish & Game.
What's coming
Plan ahead
5-day outlook
Gallery · 2
  1. A river meanders through a wide agricultural valley with a viewing bench and interpretive sign in the foreground and snow-covered mountains behind
    The Big Lost River winding through its valley near Mackay, Idaho, below the snow-capped Lost River Range
  2. Terrain map of the venue
    Terrain map
Water quality · US Clean Water Act
  • Aquatic life: Impaired
What this classification means

EPA ATTAINS assessment unit 'Big Lost River - McKay Reservoir Dam to Beck and Evan Ditch' (ID17040218SK011_05), 2022 Integrated Report. Aquatic Life use: Not Supporting. Listed impairment causes: Temperature. 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) · ID17040218SK011_05

The full read · show the working · for Brown Trout · confidence 90%
How the 83 is built — score × weight = contribution
Temperature100 × 28%28.0
Flow80 × 22%17.6
Clarity95 × 18%17.1
Feeding Time55 × 13%7.2
Pressure80 × 7%5.6
Insect activity65 × 12%7.8
Conditions total= 83
Can you trust it?
Water temperatureair-to-water estimateestimated
Level / flowon-river gaugeobserved
Prey activitymodelled from regional profilemodelled
What would change the calculation
The dawn and dusk windows score higher than the midday lull.
Directions
Apple MapsGoogle Maps
Seasons & zones
  • Trout Saturday im Memorial Day weekend → 30 November
Related guides
Euro Nymphingsubsurface methods
Booking & contacts