Big Lost River
BIG LOST RIVER
ReportJUL 30 — AUG 6, 2026
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Flow
399CFS
BIG LOST RIVER BL MACKAY RES NR MACKAY ID
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Water Temp
—
Updated 2026-07-30
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Weather
59–89°F
Mostly Sunny then Isolated Showers And Thunderstorms
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Reported By
Bruce Staples
On the water
Flow below Mackay Dam is running around 435 cfs per our staffer — confirmed elevated by USGS at 399 cfs — making wading below the reservoir dangerous and pushing trout to feed at depth on drifting invertebrates. Safer wading can be found above Mackay Reservoir where flows are closer to 340 cfs.
From the ShopMadison Losee · July 30, 2026
Flow below Mackay Dam remains about 435 cfs, which is normal for this time of the season. It is still dangerous for wading. This level of flow is scouring the stream bed such that life forms there are freed to drift, allowing foraging trout to feed at depth. Try San Juan worm and weighted nymph patterns.
The downstream flow at Leslie is about 350 cfs. Flow above Mackay Reservoir is about 340 cfs, making wading safer than below the reservoir.
What's Working — Hot Flies
Hatch Chart
| Insect | Size | Activity | Prime Time |
|---|---|---|---|
| Scoured Invertebrates / Worm Drift | #8–10 | All day — high flows dislodging bottom life | All day |
| General Nymph Drift (Hare's Ear / Frenchie types) | #12 | Consistent — trout holding deep and intercepting drift | Morning & evening |
| Stonefly Nymphs | #6–8 | Moderate — scouring action dislodges larger bottom-dwellers | Morning |
Best Time Window
- EARLY MORNING (dawn–11 a.m.): Prime nymphing window before afternoon storm threat; fish San Juan worms and weighted nymphs tight to the bottom from the bank below the dam.
- EVENING (after 6 p.m.): Storms should clear by early evening; cooler temps and calmer conditions make for a productive second session, especially above Mackay Reservoir where wading is safer.
- ABOVE MACKAY RESERVOIR (all day): Flows around 340 cfs allow safer wading — work weighted nymphs through deeper runs and seams throughout the day.
Guide's Tip
From the benchWith flows this high below Mackay Dam, do not attempt to wade — fish from the bank and use a long leader with enough split shot or a heavy anchor fly to get your rig to the bottom quickly. If you want safer footing, head above Mackay Reservoir where flows drop to around 340 cfs. Watch the afternoon sky closely: NOAA is calling for a 40% chance of thunderstorms between noon and 2 p.m. today, so plan to be off exposed water by late morning and pick back up in the evening.
Main Species
Rainbow Trout
Brook Trout
Mountain Whitefish
