Big Lost River
BIG LOST RIVER
ReportSEP 9 — 16, 2026
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Flow
304CFS
BIG LOST RIVER BL MACKAY RES NR MACKAY ID
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Water Temp
—
Updated 2026-09-09
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Weather
46–79°F
Sunny
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Reported By
Bruce Staples
On the water
Flow below Mackay Dam is running 304 cfs — wade with care; the reach at Leslie drops to roughly 240 cfs for safer footing. Above Mackay Reservoir, water is clear and cold with the best wading and dry fly conditions on the river right now.
From the ShopBruce Staples · September 9, 2026
The Big Lost River will soon round into good fishing shape as irrigation demands decrease and water storage in Mackay Reservoir begins. Flow below Mackay Dam remains about 300 cfs, which is normal for this time of the season — wade with care. The downstream flow at Leslie is about 240 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, trico, 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 (Hoppers & Ants) | #10–16 | Good — all day | Midday through afternoon |
| PMD (Pale Morning Dun) | #18–20 | Moderate | Morning into early afternoon |
| Trico | #20–24 | Moderate | Early morning |
| Caddis | #18–20 | Good — evening | Evening |
| Traditional Attractor Dries | #14–18 | Steady | All day |
Best Time Window
- EARLY MORNING (first light to 9 AM): Trico spinner falls above Mackay Reservoir — fish the flat, clear water with small dries
- MIDDAY TO AFTERNOON (10 AM to 4 PM): Hopper-dropper rigs and terrestrial dries in softer water throughout the river — PMDs also active
- EVENING (5 PM to dark): Caddis life cycle patterns — swing emergers and dead-drift adults through tailouts and slower runs
Guide's Tip
From the benchIf you have trico patterns in your box, get above Mackay Reservoir early — the clear, cold water there is fishing the best on the river right now and trico spinners can bring up fish in the morning calm. No trico patterns in our current catalog, so stock up on #20–24 trico spinners and comparaduns from your local selection before heading out. Below the dam, target the softer water edges and seams with hopper-dropper rigs rather than wading the main current at 300 cfs.
Main Species
Rainbow Trout
Brook Trout
Mountain Whitefish
