Big Lost River
BIG LOST RIVER
ReportOCT 7 — 14, 2026
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Flow
246CFS
BIG LOST RIVER BL MACKAY RES NR MACKAY ID
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Water Temp
—
Updated 2026-10-07
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Weather
46–77°F
Sunny
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Reported By
Bruce Staples
On the water
Flow below Mackay Dam is running at 246 cfs (USGS) — right in line with seasonal norms as irrigation demands ease and Mackay Reservoir begins storing water. Expect sunny skies and highs near 77°F today, with similar conditions through Thursday.
From the ShopBruce Staples · October 7, 2026
The Big Lost River is in 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.
A hopper dropper rig produces well in soft water and when drifted along vegetated banks. 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.
Flow above Mackay Reservoir is clear and cold, and much safer for wading. Try caddis life cycle patterns during evenings up there.
What's Working — Hot Flies
Hatch Chart
| Insect | Size | Activity | Prime Time |
|---|---|---|---|
| Blue-Winged Olive (Baetis) | #20–22 | Moderate — best on overcast days | Midday |
| Midge | #20–22 | Consistent throughout the day | All Day |
| Caddis | #18–20 | Active above Mackay Reservoir | Evening |
| Hopper (terrestrial) | #12 | Still producing on hopper dropper rigs | Midday–Afternoon |
Best Time Window
- MIDDAY (11 AM–2 PM): Watch for tiny BWO hatches and rising fish; nymph with small baetis and hare's ear patterns all morning leading into it
- ALL DAY: Midge hatches are consistent from first light to last — keep a zebra midge or slim midge nymph in the dropper position regardless of conditions
- EVENING (above Mackay Reservoir): Switch to caddis life cycle patterns in the clear, cold water above the reservoir for the best late-day action
Guide's Tip
From the benchToday's sunny forecast is not ideal for BWO hatches — Madison notes that overcast days bring the best mayfly numbers, so manage expectations on the dry fly front and lean on your nymph rig. If you want the best shot at rising fish, keep an eye on the sky and be ready to switch to a Parachute BWO the moment clouds roll in at midday. Above Mackay Reservoir, the water is clear, cold, and much safer to wade — plan your evening session up there with caddis life cycle patterns. The field report also mentions 'traditional patterns' broadly; if you have a go-to soft hackle or classic wet fly, don't leave it in the box.
Main Species
Rainbow Trout
Brook Trout
Mountain Whitefish
