Big Lost River
BIG LOST RIVER
ReportAUG 19 — 26, 2026
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Flow
325CFS
BIG LOST RIVER BL MACKAY RES NR MACKAY ID
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Water Temp
—
Updated 2026-08-18
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Weather
53–86°F
Mostly Clear
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Reported By
Bruce Staples
On the water
Flow below Mackay Dam is running 325 cfs — wadeable but use caution; the Leslie gauge reads 250 cfs for safer footing, and above Mackay Reservoir offers the clearest, coldest, and most wade-friendly water on the system. Sunny skies and highs in the low-to-mid 80s through Thursday set up ideal summer dry fly conditions.
From the ShopBruce Staples · August 19, 2026
Flow below Mackay Dam remains about 325 cfs, which is normal for this time of the season. It is OK for wading. The downstream flow at Leslie is about 250 cfs, making for safer wading. Flow above Mackay Reservoir — clear and cold — offers much safer wading as well as better dry fly fishing.
Terrestrial insect, PMD, and traditional dry fly patterns and hopper-dropper rigs targeting softer water can be effective throughout the day. Try caddis life cycle patterns during evenings.
What's Working — Hot Flies
Hatch Chart
| Insect | Size | Activity | Prime Time |
|---|---|---|---|
| Terrestrials (Ants, Beetles) | #12–16 | All day | Midday through afternoon |
| PMD (Pale Morning Dun) | #14–20 | All day | Morning through afternoon |
| Grasshoppers | #10–14 | All day | Midday — target softer water |
| Caddis (all life stages) | #18–20 | Evening | Late afternoon into dusk |
Best Time Window
- ALL DAY: Hopper-dropper rigs and terrestrial/PMD dry flies targeting softer water — effective from morning through late afternoon
- EVENING (late afternoon to dusk): Switch to caddis life cycle patterns as adult caddis activity picks up along the river
Guide's Tip
From the benchAbove Mackay Reservoir is your best bet for both wading comfort and dry fly action — the water is clear and cold, giving you the best shot at rising fish. Work hopper-dropper rigs along softer seams and bankside pockets throughout the day, then switch to caddis life cycle patterns as the light fades in the evening. If you're fishing below the dam near Leslie, the 250 cfs flow is more manageable, but still mind your footing on the slicker substrate.
Main Species
Rainbow Trout
Brook Trout
Mountain Whitefish
