Idaho
Big Lost River
Latest report: Sun Valley Area Fly Fishing ForecastAugust 26 – September 2 — Silver Creek Outfitters, 6 days ago
Solunar · next 7 days
About Big Lost River
The Big Lost is really two rivers wearing one name. Up top, in the Copper Basin and down the meadow reaches past Chilly and Howell Ranch, it's a small high-desert freestone winding under the Pioneer and White Knob ranges — willow-lined runs and honest wild rainbows and cutthroat that see far fewer anglers than anything over in the Wood River drainage, with a relic Arctic grayling population high in the headwaters. Then it hits Mackay Reservoir, and below the dam it turns into the thing people actually drive for: a short, cold, nutrient-rich tailwater, maybe five miles of it, that grows wild rainbows to 20 inches and better in a channel you can nearly spit across. That contrast — a technical trophy tailwater bolted onto a lonely mountain freestone — is what makes the Big Lost worth the map.
The tailwater fishes like a spring creek pretending to be a freestone. Water comes off the bottom of the dam clear and cold, the fish are educated, and the standard rig is a long leader down to 6X or 7X with small nymphs — Zebra Midges, small Baetis nymphs, sowbugs — sight-fished to specific rainbows holding in the current seams when flows drop and clear. There's dry action on Baetis, PMDs, and Tricos, but nymphing is how you consistently move fish, and locals will tell you the first trip is worth doing with a guide because reading the lies matters more than casting distance. Flows are irrigation-driven: the reservoir bleeds open through summer for downstream ag — the tailwater was running around 440 CFS in mid-July 2026, and Howell Ranch above the lake about 355 — then drops hard in fall, which is when the wading gets easy and the fishing gets good. Late fall into winter is prime on the lower river: low, clear water, BWOs and midges, the biggest rainbows, and almost nobody out there.
The quirk that defines the whole system: the Big Lost doesn't reach any ocean. Below Arco it spreads across the desert and sinks into the Snake River Aquifer at the Big Lost River Sinks, which is why the USGS gauge near Arco legitimately reads 0.00 CFS through the summer irrigation season — that's the river working as designed, not a dead station. Bring everything you need before Mackay; there are no services up the valley, and the upper reaches cross a lot of private ranch ground, so mind access.
Species
- Rainbow Trout
- Yellowstone Cutthroat Trout
- Brook Trout
- Brown Trout
- Arctic Grayling
- Mountain Whitefish
| Species | Abundance | Best Season | Size |
|---|---|---|---|
| Rainbow Trout | Primary | Oct-Mar, Jul-Aug | 14-21" |
| Yellowstone Cutthroat Trout | Common | Jun-Sep | 8-16" |
| Brook Trout | Present | Jun-Sep | 6-12" |
| Brown Trout | Present | Sep-Nov | 12-18" |
| Arctic Grayling | Present | Jun-Sep | 8-13" |
| Mountain Whitefish | Abundant | Year-round | 8-16" |