Big Lost River
BIG LOST RIVER
ReportOCT 21 — 28, 2026
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Flow
75CFS
BIG LOST RIVER BL MACKAY RES NR MACKAY ID
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Water Temp
—
Updated 2026-10-20
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Weather
41–61°F
Partly Cloudy then Slight Chance Light Rain
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Reported By
Bruce Staples
On the water
The USGS gauge below Mackay Reservoir is reading 74.7 cfs with a gauge height of 1.6 ft, making wading very safe throughout the system. Overcast skies in the forecast through Thursday set up ideal conditions for the midday BWO hatches Madison reported.
From the ShopBruce Staples · October 21, 2026
Flow below Mackay Dam is about 90 cfs, making wading very safe. The downstream flow at Leslie is about 80 cfs. Trout are concentrated in deeper runs and holes. Tiny BWOs hatch during midday while midge hatches go on throughout the day. Overcast days have the best numbers of BWOs emerging. Frosts have diminished the number of bankside terrestrial insects.
Small nymph patterns can provide day-long action but are most effective early in the day before trout start rising to hatching BWOs. Flow above Mackay Reservoir is clear and cold, and is much safer for wading. Try October caddis patterns as evening approaches.
What's Working — Hot Flies
Hatch Chart
| Insect | Size | Activity | Prime Time |
|---|---|---|---|
| Blue-Winged Olive (Baetis) | #22 | Active — best on overcast days | Midday |
| Midge | #20–24 | Active | All day |
| October Caddis | #12 | Moderate | Evening |
Best Time Window
- EARLY MORNING (first light to ~10 am): Nymph small patterns like the Juju Baetis or Zebra Midge through deeper runs and holes where trout are concentrated
- MIDDAY (10 am to 3 pm): Watch for rising fish and switch to BWO dries or emergers — overcast conditions forecast for Wednesday will produce the strongest hatches
- EVENING (last 1–2 hours of light): Swing or dead-drift October caddis patterns, particularly above Mackay Reservoir
Guide's Tip
From the benchTime your day in three acts: start early with small nymphs in the deeper runs and holes, then watch for rising fish and switch to BWO dries or emergers as the midday hatch develops — overcast skies like those forecast for Wednesday will push the best numbers of bugs. As the light drops in the evening, tie on an October caddis pattern, especially if you're fishing above Mackay Reservoir where the water is clear and cold. Terrestrials are largely done for the season after recent frosts, so don't waste time searching banks with hoppers or ants.
Main Species
Rainbow Trout
Brook Trout
Mountain Whitefish
