Idaho Fly Fishing 7 min read
Fly Fishing Big Lost River, Idaho: Map & DIY Guide

Nestled within the stunning landscapes of central Idaho, the Big Lost River provides an exceptional fly fishing experience for anglers seeking solitude and the excitement of catching wild trout. Flowing through rugged canyons and open meadows, this river offers diverse fishing opportunities amidst breathtaking scenery.
Contents
- About the Big Lost River
- Big Lost River Map
- Where to Go Fly Fishing on the Big Lost River
- Upper Big Lost River
- Upper River Access Points
- Upper River Fly Fishing Strategies
- Upper River Seasonal Hatches
- Lower Big Lost River Tailwater
- Big Lost River Tailwater Access and Locations
- Tailwater Fishing Techniques and Tips
- Big Lost River Tailwater Hatches and Flies
- Best Times to Fish Big Lost River Tailwater
- Equipment Recommendations
- Best Times to Fish the Big Lost River
- Fly Recommendations
- Fishing Tips for Success
- Big Lost River Fishing Report
- Stream Flow and Current Conditions
- Regulations
- Amenities and Accommodations
- Safety Considerations
About the Big Lost River
The Big Lost River starts in the Pioneer Mountains. It travels approximately 135 miles before vanishing into the Snake River Aquifer near Arco, Idaho—a unique characteristic that gives the river its name. The river system comprises several tributaries and is split by the Mackay Reservoir into an upper and lower river, each offering distinct fishing experiences.
Big Lost River Map
Before heading out to fish, you’ll need a good map of the Big Lost River. We’ve got you covered with the DIY Fly Fishing map and mobile app. All the best places to fish the Big Lost River are included on the map, so you can spend more time fishing and less time worrying about where the best spots are.

Get Directions to the Fishing Access Points shown above with the DIY Fly Fishing Map
Where to Go Fly Fishing on the Big Lost River
When searching for prime fishing spots on the Big Lost River, anglers should focus on accessible locations and consider whether they prefer the more remote upper river or the tailwater section below Mackay Reservoir. Each choice offers unique advantages that cater to different fishing styles and preferences.
Upper Big Lost River
- Location: Above Mackay Reservoir
- Features: Meandering streams, lush meadows, and forested areas
- Target Species: Wild rainbow trout, brook trout, and cutthroat trout
- Fishing Style: Perfect for small stream fishing using dry flies and nymphs
- Access Points: Accessible via Trail Creek Road and various pull-offs along Highway 93
The convergence of two main branches forms the Upper Big Lost River:
- East Fork: Renowned for its wild rainbow and cutthroat trout populations, it presents a more challenging yet rewarding experience. Special regulations are in place to maintain its status as a wild trout fishery.
- North Fork: This branch is more accessible and stocked in certain areas, making it suitable for anglers of all skill levels.
After the forks merge, the river’s main stem continues through the Lost River Valley, characterized by sagebrush plains and stunning mountainous backdrops with peaks exceeding 12,000 feet.
Upper River Access Points
Access to the upper Big Lost River is generally good. Trail Creek Road provides routes to various sections:
- North Fork: Offers easier access, ideal for beginners and those seeking a more relaxed fishing experience.
- East Fork: Accessing this branch may require traveling on rough dirt roads, but the secluded spots are well worth the effort.
Upper River Fly Fishing Strategies
- East Fork Tips: Stealth and careful presentation are crucial due to the clear water and cautious wild trout. Nymphing techniques with patterns that match local aquatic insects can be highly effective.
- North Fork Tips: Suitable for dry fly fishing, especially during hatch seasons. Stocked trout here provide an excellent opportunity for beginners or those looking for a more relaxed fishing experience.
- Main Stem Tips: While trout numbers may be lower, the peaceful environment makes it a pleasant area to explore. Streamers can be useful in deeper pools where larger trout may reside.
Upper River Seasonal Hatches
Understanding the river’s insect activity enhances success on the water:
Spring (May – June)
- Blue-Winged Olive Mayflies: Among the first to hatch as temperatures rise.
- Stoneflies: Including Salmonflies and Golden Stoneflies, their emergence provides exciting fishing opportunities.
Summer (June – August)
- Pale Morning Duns: Abundant during early summer, matching this hatch essential.
- Caddisflies: Various species become active, making caddis patterns effective.
- Terrestrials: Grasshoppers, ants, and beetles become important food sources for trout.
Fall (September – October)
- Blue-Winged Olive Mayflies: A second emergence occurs, offering late-season action.
- Midges: As other hatches wane, midges become a staple in the trout’s diet.
Lower Big Lost River Tailwater
- Location: Below Mackay Dam
- Features: Tailwater fishery with consistent flows and temperatures
- Target Species: Larger rainbow trout and occasional brown trout
- Fishing Style: Requires precise presentations with a mix of nymphs, dry flies, and streamers
- Access Points: Accessed via parks and pull-outs near Mackay and downstream along the river
The tailwater section is known for its clear, cold waters and abundant aquatic life, providing an ideal habitat for trout. Anglers can expect wild rainbow trout averaging 14 to 20 inches, with some larger fish. These strong and wary trout offer a challenging yet fulfilling fishing experience. Additionally, the river supports brook, cutthroat, and occasionally brown trout populations in certain areas and mountain whitefish.
Big Lost River Tailwater Access and Locations
Access to the Big Lost River tailwater is limited. The two primary accesses are off Highway 93, at the dam’s base, and 2 miles downstream, just south of the Trout Haven Campground. The river ranges from 20 to 30 feet wide, making it suitable for wading. Always respect private property boundaries and use designated access areas. Where the river flows through private land, stay below the high water mark to stay legal.
Tailwater Fishing Techniques and Tips
The Big Lost River Tailwater presents technical fishing conditions due to its clear water and educated trout. Success relies on a stealthy approach and precise presentations.
Nymphing
- Nymph fishing is effective year-round.
- Small mayflies and midge nymphs drifting near the riverbed attract feeding trout.
- Use light tippets (5X to 7X) and indicators for subtle takes.
Dry Fly Fishing
- During hatches, dry fly fishing can be productive.
- Matching the hatch with appropriate patterns is crucial.
- Use long leaders and delicate presentations to increase success.
Sight Fishing
- On sunny days with low flows, sight fishing for visible trout adds excitement.
- Polarized sunglasses aid in spotting fish, and careful stalking is essential.
Streamer Fishing
- Early mornings or overcast days are ideal for streamer fishing.
- Casting streamers into deep pools or undercut banks can provoke aggressive strikes from larger trout.
Big Lost River Tailwater Hatches and Flies
Understanding the river’s hatch cycles enhances the fishing experience.
- Blue-Winged Olives: Hatch in spring and fall. Use BWO dry flies and emergers in sizes 18-22.
- Pale Morning Duns: Hatch from late spring through midsummer. PMD patterns in sizes 16-18 are effective.
- Caddisflies: Various species hatch from late spring into fall. Standard Elk Hair Caddis in sizes 14-18 work well.
- Midges: Present year-round, especially in colder months. Use midge larva, pupa, and adult patterns in sizes 20-24.
- Stoneflies: Golden stoneflies and occasional salmonflies appear in early summer. Nymph patterns in sizes 8-12 are helpful.
- Terrestrials: In summer, grasshoppers, ants, and beetles become significant. Terrestrial patterns fished close to the banks can be very effective.
Best Times to Fish Big Lost River Tailwater
The prime fishing season runs from late June through September after spring runoff subsides and flows stabilize. Water levels may fluctuate due to irrigation demands, so checking current flow conditions is advisable. Winter fishing is possible on warmer days, with midges providing the main hatches. Be prepared for cold temperatures and potentially challenging conditions.
Equipment Recommendations
- Rod: A 9-foot, 4 to 6-weight rod covers most situations on the Big Lost River.
- Reel: Match your reel to the rod weight, ensuring it has a smooth drag system to handle strong trout.
- Line: Weight-forward floating lines are standard. Consider a sinking tip line for streamers fishing in deeper pools.
- Leaders and Tippets: Use long leaders (9 to 12 feet) with fine tippets (5X to 7X) for delicate presentations.
- Waders and Boots: Chest waders and boots with good traction are recommended due to the river’s cool temperatures and slippery substrates.
Best Times to Fish the Big Lost River
- Spring: Runoff can raise water levels, making fishing more challenging. Nymphing with stoneflies and mayfly patterns is effective.
- Summer: The prime fishing season with excellent hatches of caddisflies, mayflies, and terrestrials like grasshoppers. Dry fly fishing is particularly rewarding.
- Fall: Cooler temperatures and lower water levels drive trout into a feeding frenzy before winter. Streamer fishing can help land larger trout.
- Winter: The lower river remains fishable. Nymphing is preferred, and anglers should be prepared for cold conditions.
Fly Recommendations
- Dry Flies: Parachute Adams, Elk Hair Caddis, PMDs, and hopper patterns during summer
- Nymphs: Pheasant Tail, Hare’s Ear, Prince Nymph, and Midge patterns
- Streamers: Woolly Buggers, Zonkers, and sculpin patterns for targeting larger trout
Fishing Tips for Success
- Stealth and Presentation: Clear waters require a cautious approach. Use light tippets and make gentle casts to avoid spooking fish.
- Reading the Water: Look for riffles, pools, and undercut banks where trout will likely hold.
- Insect Activity: Pay attention to hatches and match your fly selection accordingly. Afternoon hatches can be particularly prolific.
Big Lost River Fishing Report
Area fly shops, guides, and websites that can provide a Big Lost River fly fishing report and update on current conditions are listed below:
Stream Flow and Current Conditions
Check the stream conditions before heading out to fish the Big Lost River. A series of USGS stream gauges from the Upper Big Lost River headwaters to below the MacKay dam indicate current conditions.
The graphs below show the stream flow (discharge) for the past seven days. If flows are considerably above or below historical norms (yellow triangles on the chart), fishing conditions may not be ideal.
North Fork Big Lost River
Upper Big Lost River
Lower Big Lost River (below MacKay Reservoir)
Regulations
The fishing regulations for Big Lost River can be found on the Idaho Fish and Game website.
- Licensing: A valid Idaho fishing license is required for anglers aged 14 and over.
- Limits: December 1 through Friday before Memorial Day weekend is catch-and-release for trout and whitefish. Saturday of Memorial Day weekend through November 30, the trout limit is 6, and all whitefish must be released.
- Conservation: Practice catch-and-release whenever possible to help maintain healthy trout populations.
Amenities and Accommodations
- Mackay: The nearest town offers lodging, dining, and basic supplies. Make sure you have all the supplies you need before venturing past Mackay, as there are no services past this point.
- Camping: Several campgrounds are available in the area, including those managed by the U.S. Forest Service.
Safety Considerations
- Weather: Idaho’s mountain weather can change rapidly. Dress in layers and be prepared for sudden changes.
- Wildlife: Be aware of local wildlife, including bears and moose. Carry bear spray and know how to use it.
- Navigation: Cell service may be limited. Carry a map or GPS device when exploring remote areas.
The Big Lost River is a gem for fly anglers seeking a peaceful and rewarding fishing adventure. With its diverse insect hatches, variety of trout species, and stunning scenery, it offers something for every fly fishing enthusiast. Plan your trip, respect the natural environment, and enjoy all that this remarkable river offers.
Looking for more places to fish? Check out our DIY Guide to the Best Fly Fishing in Idaho.