Big Lost River
BIG LOST RIVER
ReportAUG 4 — 11, 2026
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Flow
394CFS
BIG LOST RIVER BL MACKAY RES NR MACKAY ID
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Water Temp
—
Updated 2026-08-04
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Weather
51–85°F
Areas Of Smoke
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Reported By
Bruce Staples
On the water
Flow below Mackay Dam is running around 435 cfs — too high for safe wading — scouring the streambed and pushing invertebrates into the drift, with trout keying on depth. Above the reservoir, flows are dropping and wading is becoming more manageable.
From the ShopBruce Staples · August 4, 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 scours the streambed 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 dropping, making wading safer than below the reservoir.
What's Working — Hot Flies
Hatch Chart
| Insect | Size | Activity | Prime Time |
|---|---|---|---|
| General Invertebrate Drift (scoured streambed) | #8–14 | All day | Midday through afternoon |
| Midges | #20–22 | Low — trout focused subsurface | Morning & evening |
| Caddis | #16–18 | Low — trout focused subsurface | Evening |
Best Time Window
- MIDDAY TO AFTERNOON: Fish weighted nymphs and worm patterns deep along the bank from safe, stable positions below Mackay Dam.
- ALL DAY ABOVE THE RESERVOIR: Dropping flows make wading safer up top — work weighted nymphs through seams and runs where drifting invertebrates concentrate.
- EVENING AT LESLIE: With flows around 350 cfs, target the Leslie reach with a two-nymph rig anchored by a heavy fly to stay in the strike zone.
Guide's Tip
From the benchWith flows at 435 cfs below Mackay Dam, wading is dangerous — do not attempt it. Fish from the bank or a stable position and use enough weight to get your San Juan worm or nymph all the way to the bottom where the trout are holding. Above Mackay Reservoir, dropping flows are making access safer and worth exploring. If you want a true San Juan Worm pattern, ask us at the shop — we can point you to the right match.
Main Species
Rainbow Trout
Brook Trout
Mountain Whitefish
