Big Lost River
BIG LOST RIVER
ReportSEP 30 — OCT 7, 2026
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Flow
246CFS
BIG LOST RIVER BL MACKAY RES NR MACKAY ID
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Water Temp
—
Updated 2026-09-30
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Weather
45–73°F
Sunny
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Reported By
Bruce Staples
On the water
Flow below Mackay Dam is running around 246 cfs (USGS) — normal for late September as irrigation demands ease and Mackay Reservoir begins storing water. Wading below the dam requires care; the reach above the reservoir is clear, cold, and much safer underfoot.
From the ShopBruce Staples · September 30, 2026
The Big Lost River is rounding into good fishing shape as irrigation demands decrease and water storage in Mackay Reservoir begins. Flow below Mackay Dam is about 250 cfs, which is normal for this time of the season — wade with care. The downstream flow at Leslie is about 190 cfs.
Tiny BWOs hatch during midday while midge hatches go on throughout the day. Overcast days have the best numbers of mayflies emerging. Small baetis, zebra and hare's ear nymph, and traditional patterns can provide day-long action. Terrestrial insects are present in good numbers on vegetated banks.
Flow above Mackay Reservoir is clear and cold, and is much safer for wading. Try caddis life cycle patterns during evenings.
What's Working — Hot Flies
Hatch Chart
| Insect | Size | Activity | Prime Time |
|---|---|---|---|
| Midge | #20–22 | Active all day | All day |
| Blue-Winged Olive (Baetis) | #20–22 | Midday hatch; best on overcast days | 10 AM – 2 PM |
| Caddis | #18–20 | Evening activity, especially above Mackay Reservoir | Evening |
| Terrestrials (ants, hoppers) | #12–16 | Present in good numbers on vegetated banks | Midday – Afternoon |
Best Time Window
- MIDDAY (10 AM – 2 PM): Focus on the BWO hatch with a Parachute BWO or small baetis nymph; overcast days will produce the best numbers of rising fish.
- ALL DAY: Midge hatches run continuously — keep a zebra midge or small nymph in the water from first light through last light for consistent action.
- EVENING: Switch to caddis life cycle patterns above Mackay Reservoir where wading is safer and the water is clear and cold.
Guide's Tip
From the benchToday's sunny skies (high near 72°F) will likely suppress the BWO hatch — don't abandon the midday window entirely, but lean on your midge and nymph rigs until clouds roll in. If you're chasing the caddis evening bite, head above Mackay Reservoir where the water is clear, cold, and far easier to wade safely. The staffer also mentions 'traditional patterns' as day-long producers — classic soft hackles and hare's ear variants are worth keeping on a dropper all day.
Main Species
Rainbow Trout
Brook Trout
Mountain Whitefish
