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

The Big Lost is a desert tailwater below Mackay Reservoir, and it earns its name — downstream it disappears into the lava sinks of the Snake River Plain, going nowhere on the map at all.

Tomorrow · About as good as it gets
Czech Nymph
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
Czech Nymph
Czech Nymph
Conditions on the water · Tomorrow
Live gauge
Level
Steady
0.72 m
Water temp11.5°C
ClarityClear
Weather13°C
WindN 8 km/h
Pressure1014 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
12°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 working for you, but time of day is the limiting factor today.
Likely species
Mayflies
Caddisflies
terrestrials
Hatch timeline · tomorrowPeak at 11am

12pm(now) — Western Blue-Winged Olive

Stage: On emergersApproach: Emerger or dryActivity: 87%
Also:Black Midge53%
Peak emergence windowWeather boost8 food groups active

Peak activity at 11am — Western Blue-Winged Olive (Western Blue-Winged Olive, 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 3 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
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
Brown Trout fly box
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
About this water

The Big Lost is a desert tailwater below Mackay Reservoir, and it earns its name — downstream it disappears into the lava sinks of the Snake River Plain, going nowhere on the map at all. The river below the dam holds surprisingly good rainbows and a few big browns in cold, clear water, though the flow is entirely at the mercy of irrigation demand and can change overnight. It's lightly fished, out on its own in big sagebrush country. When the release is right it's a quiet little gem; when the farms need water it's a different river.

  • Mixed
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 75%
How the 81 is built — score × weight = contribution
Temperature100 × 28%28.0
Flow80 × 22%17.6
Clarity95 × 18%17.1
Feeding Time40 × 13%5.2
Pressure80 × 7%5.6
Insect activity65 × 12%7.8
Conditions total= 81
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 of Memorial Day weekend → 30 November
Related guides
Euro Nymphingsubsurface methods
Booking & contacts