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  3. /Big Lost River

Idaho

Big Lost River

Idaho·Central Idaho·43.94° N, 113.65° W
Log an outing

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
    Primary · Oct-Mar, Jul-Aug · 14-21"

    The tailwater draw — wild fish average 14 to 20 inches with some to 25. Educated and sight-nymphed on 6X to 7X below Mackay Dam; also the mainstay of the upper freestone reaches.

  • Yellowstone Cutthroat Trout
    Common · Jun-Sep · 8-16"

    The upper river and Copper Basin fish. Notably one of the few Idaho streams where cutthroat may legally be harvested within the trout limit — an unusual regulation for the state.

  • Brook Trout
    Present · Jun-Sep · 6-12"

    Headwater tributaries and the East and North Fork meadows. Small but eager for attractor dries in the tumbling pocket water.

  • Brown Trout
    Present · Sep-Nov · 12-18"

    Not abundant — an occasional streamer target on the lower and mainstem reaches in fall.

  • Arctic Grayling
    Present · Jun-Sep · 8-13"

    A relic, introduced population high in the Copper Basin and East Fork headwaters — a genuine oddity for this part of the West and a reason to hike into the upper drainage.

  • Mountain Whitefish
    Abundant · Year-round · 8-16"

    Native and everywhere. Must be released Dec 1 through the Friday before Memorial Day; catch-and-keep otherwise but usually released.

SpeciesAbundanceBest SeasonSize
Rainbow TroutPrimaryOct-Mar, Jul-Aug14-21"
Yellowstone Cutthroat TroutCommonJun-Sep8-16"
Brook TroutPresentJun-Sep6-12"
Brown TroutPresentSep-Nov12-18"
Arctic GraylingPresentJun-Sep8-13"
Mountain WhitefishAbundantYear-round8-16"

Sections

4 reaches

Upper Mainstem — Chilly / Howell Ranch to Mackay Reservoir

WadeCutthroat · Rainbow Trout

The mainstem meanders through open ranch meadows below the Chilly and Howell Ranch flats, wider than the forks with deeper runs and undercut banks, before slowing into the head of Mackay Reservoir. Much of it crosses private ranch ground, so mind boundaries. Wild rainbow trout and cutthroat trout come to hoppers, attractor dries, and nymphs; the Howell Ranch USGS gauge is the above-reservoir flow signal.

Best for: Wild rainbow trout and cutthroat trout on hoppers, attractor dries, and nymphs through open meadow water.

North Fork (Wild Horse)

WadeCutthroat · Rainbow Trout

A freestone tributary through meadow and canyon that joins the mainstem near Chilly, holding its own USGS gauge at Wild Horse. Small, clear water with wild rainbow trout and cutthroat trout eager for a well-drifted dry or a light nymph.

Best for: Wild rainbow trout and cutthroat trout on dry-dropper and light nymph rigs in small, wadeable water.

Mackay Tailwater — Below Mackay Dam

WadeRainbow Trout

The signature fishery — roughly five miles of cold, clear, nutrient-rich bottom-release water 20 to 30 feet wide, fishing like a spring creek pretending to be a freestone. Educated wild rainbow trout hold in specific current seams and are sight-nymphed on long leaders down to 6X or 7X with small Baetis, midge, sowbug, and PMD nymphs. Access is two Highway 93 pullouts — the dam base and a spot near Trout Haven Campground about two miles down — plus Lost River Outfitters' private lease below. Wading gets easy once fall flows drop. Below here the river runs high-desert to Arco and disappears into the Big Lost River Sinks, losing water to irrigation until it goes dry by mid-summer — which is why the USGS gauge near Arco legitimately reads 0.00 cfs through the irrigation season. That lower reach is a marginal, early-season-only proposition where water still runs; the tailwater is the fishery.

Best for: Trophy wild rainbow trout — technical sight-nymphing on long leaders and fine tippet; some dry-fly to Baetis and PMDs.

Copper Basin & East Fork (Headwaters)

WadeCutthroat · Brook Trout · Grayling · Rainbow Trout

High freestone meadow streams from 7,800 to 9,500 feet, where the East Fork gathers in the broad Copper Basin depression under 12,000-foot peaks. Small willow-lined pocket water and riffles, cold and clear, with wild cutthroat trout and brook trout — plus a relic Arctic grayling population high in the drainage that is a genuine oddity for the region.

Best for: Backcountry cutthroat trout, brook trout, and headwater Arctic grayling on dry-dropper rigs and attractor dries; solitude far from the Wood River crowds.

At a glance

Ideal wading flow
60–250 cfs
Blow-out
>800 cfs
Ideal water temp
48–60°F
Best seasons

Late fall and winter are the top window on the Mackay tailwater — the reservoir cuts flows, the water drops and clears, and low, clear conditions bring out BWOs and midges over the biggest rainbows with the fewest anglers. Summer is the upper-river season: dries, hoppers, and cutthroat and grayling on the freestone meadows once runoff clears. The tailwater shines for wading and sight-fishing roughly between 60 and 250 CFS; summer irrigation releases around 400 to 450 CFS push it high and make sight-nymphing harder. The upper freestone blows out during peak snowmelt, typically May into early June, and the lower river below Arco goes dry by design in mid-summer.

Regulations

General-rule trout water with a seasonal catch-and-release window. From December 1 through the Friday before Memorial Day the whole river is catch-and-release for trout and whitefish; from the Saturday of Memorial Day weekend through November 30 the trout limit is six and all whitefish must be released. Unusually for Idaho, Yellowstone cutthroat may be kept within the trout limit on the upper river.

  • Dec 1 – Friday before Memorial Day: catch-and-release only for trout and whitefish
  • Saturday of Memorial Day weekend – Nov 30: trout limit 6
  • All whitefish must be released during the general season (Memorial Day – Nov 30)
  • Yellowstone cutthroat may be harvested within the trout limit — one of the few Idaho streams that allows it
  • Idaho fishing license required (age 14+)

No fly-only or barbless-only designation applies to the mainstem — standard general rules govern. Idaho rules change annually, so verify the current-year regulation booklet before your trip, particularly the seasonal catch-and-release dates.

Source: Idaho Department of Fish and Game — Fishing Planner. Regulations change annually. Verify before fishing.

Getting there

Mackay, ID

About 1.5 hr from Idaho Falls (IDA), 1 hr from Sun Valley/Hailey (SUN) over Trail Creek Road in summer, and roughly 3 hr from Boise

Fly shops

  • Lost River OutfittersKetchum
  • Silver Creek OutfittersKetchum
  • Picabo AnglerPicabo
  • Jimmy's All-Seasons AnglerIdaho Falls
  • Sturtevants of Sun ValleyKetchum

Camping: No dedicated fly-fishing lodge sits on the Big Lost — lodging is motels in Mackay (at the reservoir) and Arco, plus the full services of Ketchum and Sun Valley about an hour west over Trail Creek Road in summer. Camping is at Trout Haven Campground by the tailwater access two miles below the dam, the BOR campground at Mackay Reservoir, and dispersed sites up Copper Basin and the upper valley.

There are no services past Mackay heading up the valley — fuel, food, and flies before you go. Below the dam, public access is limited to two Highway 93 pullouts (the dam base and near Trout Haven), with Lost River Outfitters leasing a couple of miles of private water below; much of the upper mainstem crosses private ranch ground, so mind boundaries.

Fishing reports

Recent reports from local fly shops, guides & lodges on Big Lost River. Each links out to the source. We show a short attributed excerpt and never republish the full report.

Silver Creek Outfitters · Ketchum9 days ago
Sun Valley Area Fly Fishing ForecastOctober 7 – October 14

““It is good to have an end to journey towards; but it is the journey that matters, in the end.” – Ursula K. Le Guin The trout give the journey direction, but the pursuit is what keeps you coming back. Silver Creek When the cool weather returns this weekend, it will concentrate…”

Picabo Angler · Picabo2 weeks ago
Fishing Report – 9/25/26

“Silver Creek continues to be productive. Baetis (BWOs), Mahogany Duns, and October Caddis are the bugs du jour. There’s no need to rush to the creek, as the activity really gets going during the late morning and early afternoon. The BWOs will be small, with a size 18 being…”

Silver Creek Outfitters · Ketchum3 weeks ago
Sun Valley Area Fly Fishing ForecastSeptember 23 – 30

“Photo: Bob Knoebel “Spend the afternoon, you can’t take it with you.” – Annie Dillard An autumn afternoon on a river is something you can hold in your heart even after the cold water running past your legs is gone. Silver Creek Fall has arrived on the Creek. You can hear elk…”

Picabo Angler · Picabo3 weeks ago
Fishing Report – 9/18/26

“Silver Creek comes alive during autumn! The fall bugs will hatch during the most pleasant part of the day, and the surrounding scenery on the creek is breathtaking. We’re seeing excellent hatches of small Baetis/BWO (size 20 – 24) on the entirety of Silver Creek. The October…”

Picabo Angler · Picabo4 weeks ago
Fishing Report – 9/11/26

“Fishing on area waters remains very good. Cooler nighttime temperatures will put fish on the move and have them feeding in earnest in preparation for fall spawning and the harsher conditions of winter. Now is the time to hunt trophy trout – big browns on Silver Creek, 20-inch…”

Picabo Angler · Picabo5 weeks ago
Fishing Report – 9/4/26:

“September marks the unofficial start of fall in the Wood River Valley, and with it comes some of the season’s finest fishing. While most of our waters are low, recent cooler nighttime temperatures have provided a needed respite from the heat and have us thinking about fall…”

Silver Creek Outfitters · Ketchum6 weeks ago
Sun Valley Area Fly Fishing ForecastSeptember 2 – 9

““If you want the rainbow, you have to put up with the rain.” – Dolly Parton A beautiful, bright sun, windless day could mean tough fishing. But if blustery weather is in the forecast, head to the river. Silver Creek The possibility of rain and cooler weather is in the forecast.…”

Picabo Angler · Picabo6 weeks ago
Fishing Report – 8/28/26:

“We have some mild weather in the forecast, so expect to see good fishing conditions on our local waters. The Kilpatrick Bridge replacement project started on August 24th, so anglers intending to fish the Nature Conservancy Silver Creek Preserve must access the property via…”

Silver Creek Outfitters · Ketchum7 weeks ago
Sun Valley Area Fly Fishing ForecastAugust 26 – September 2

“Photo: Hillary Maybery “Life starts all over again when it gets crisp in the fall.” – F. Scott Fitzgerald The rivers continue to drop, cool, and clear. The trout are becoming more aggressive, feeding on revitalized late-season hatches. The foliage is beginning to turn from…”

Picabo Angler · Picabo7 weeks ago
Fishing Report – 8/21/26

“On Silver Creek, the Trico hatch continues, although it has been more sporadic of late. Also, with recent cooler mornings, the hatch has started a bit later. Small (size 22-24) BWOs have also been hatching in the morning, and fish will actively key in on these bugs. Believe it…”

Picabo Angler · PicaboAug 14, 2026
Fishing Report – 8/14/26:

“A recent cool spell has brought respite from the summer heat, with mild daytime temperatures and cool, crisp nights. A bit of rain, along with cooler temperatures, has helped refresh our local waters. Mornings at Silver Creek have been good, with BWOs and Tricos the main…”

Silver Creek Outfitters · KetchumAug 11, 2026
Sun Valley Area Fly Fishing ForecastAugust 12-19

“Photo: Hillary Maybery “Rhythm was just as important as color and just as complicated. It was one rhythm superimposed upon another, our father’s four-count rhythm of the line and wrist being still the base rhythm… The canyon was glorified by rhythms and colors.” – Norman Maclean…”

Picabo Angler · PicaboAug 7, 2026
Fishing Report – 8/7/26

“As we usher into August, we look forward to fishing the Hopper hatch. As grasshoppers mature, they become an important food source for trout on our local fisheries. Windy summer afternoons can provide some excellent Hopper fishing on Silver Creek, the Big Wood River, the lower…”

Silver Creek Outfitters · KetchumAug 4, 2026
Sun Valley Area Fly Fishing ForecastAugust 5-12

“Photo: Hillary Maybery “Music washes away from the soul the dust of everyday life.” – Berthold Auerbach For anglers, the river’s symphony washes from the soul the dust of everyday life — the swell and hush of the current, the riffle’s rush of white noise, the promise of a…”

Picabo Angler · PicaboJul 31, 2026
Fishing Report – 7/31/26

“The Trico hatch is in full swing on Silver Creek. Be sure to have dun and spinner Trico imitations, long leaders of 12 feet or more (6X and 6.5X), and a solid downstream presentation when fishing this hatch. Baetis (BWOs) have been mixed in with Tricos, so be prepared to switch…”

Picabo Angler · PicaboJul 24, 2026
Fishing Report – 7/24/26

“The recent heat wave has shifted fish, bug, and angler activity to the coolest part of the day; the hours around sunrise. Right now, it pays to be an early bird and to watch the sunrise on Silver Creek. The Trico hatch is in full swing on Silver Creek. Be sure to have Trico dun…”

Silver Creek Outfitters · KetchumJul 21, 2026
Sun Valley Area Fly Fishing ForecastJuly 22-29

““To put your hands in a river is to feel the chords that bind the earth together.” – Barry Lopez When you wade a river for a season, it begins to shape your imagination. You become connected to its vicissitudes and your heart ripples to its iron strings. The water’s rhythm gets…”

More in Idaho

View all 17 rivers

Central Idaho

Middle Fork Salmon River

507 cfs

Fair

Salmon River

472 cfs49°F water

Good

Other regions

Big Wood River

53.5 cfs

Fair

Boise River

249 cfs58.9°F water

Good

Clearwater River

3,180 cfs52°F water

Prime

Fall River

443 cfs

Good

Henry's Fork

0.25 cfs50.7°F water

Fair

Lochsa River

376 cfs

Fair
Fishing
Good
Flow
180cfs
steady · Big Lost River below Mackay Reservoir near Mackay
Water temp
—
Flow vs normal
Above Normal
65th percentile for mid-October
Flow
180cfs
Big Lost River below Mackay Reservoir near Mackay · stable
Water temp
—
Flow vs normal
Above Normal
65th percentile for mid-October
Median 153 cfs
Weather
39°F
Clear · near Mackay

Fishing conditions

Good
PoorSlowFairGoodPrime

Latest report: Sun Valley Area Fly Fishing ForecastOctober 7 – October 14 from Silver Creek Outfitters, 10 days ago

How we rate this
Sat
10/17
—
MDE
Sun
10/18
—
MDE
Mon
10/19
—
MDE
Tue
10/20
—
MDE
Wed
10/21
—
MDE

MmorningDmiddayEevening

poorprime

Today
morning6:51a – 11:51aGood
midday11:51a – 2:51pGoodRising pressure
evening2:51p – 7:51pGoodFalling pressure
All day
Bright sun
Measured
180 cfs, in range

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