Big Lost River
BIG LOST RIVER
ReportSEP 1 — 8, 2026
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Flow
316CFS
BIG LOST RIVER BL MACKAY RES NR MACKAY ID
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Water Temp
—
Updated 2026-09-01
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Weather
44–73°F
Scattered Showers And Thunderstorms
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Reported By
Bruce Staples
On the water
Flow below Mackay Dam is running around 325 cfs — wade with care in that stretch, while the downstream reach at Leslie (~250 cfs) and the water above Mackay Reservoir offer safer wading and better dry fly opportunities. Afternoon thunderstorms are possible today, with conditions improving through Wednesday.
From the ShopBruce Staples · September 1, 2026
The Big Lost River will soon round into good fishing shape as irrigation water demands decrease and water storage in Mackay Reservoir begins. Flow below Mackay Dam remains about 325 cfs, which is normal for this time of the season, but will drop to safer levels as water storage begins in the reservoir above. Wade with care.
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 (Hoppers & Ants) | #10–16 | High | All day, softer water |
| PMD (Pale Morning Dun) | #18–20 | Moderate | Midmorning through afternoon |
| Caddis | #18–20 | Moderate | Evening |
Best Time Window
- ALL DAY: Fish hopper-dropper rigs and PMD dries in softer water, especially above Mackay Reservoir and at the Leslie reach
- EVENING: Switch to caddis life cycle patterns — dries, emergers, and soft hackles — as caddis activity picks up after the heat of the day
- MIDMORNING–AFTERNOON: Best window for PMD dry fly action in the clearer, colder water above Mackay Reservoir
Guide's Tip
From the benchWith flows still elevated below Mackay Dam, focus your wading efforts on the reach at Leslie or above the reservoir where the water is clear, cold, and much more manageable. Work hopper-dropper rigs with a PMD nymph dropper through softer seams and along cut banks during the day, then switch to caddis life cycle patterns — including pupae and soft hackles — as the light fades in the evening. If the field report's mention of 'traditional dry fly patterns' has you reaching for a classic Elk Hair Caddis or Parachute Adams, those are solid choices not listed here — ask us at the shop for what's in stock.
Main Species
Rainbow Trout
Brook Trout
Mountain Whitefish
