Draining Idaho’s tallest peaks in the Lost River Range, the Big Lost River begins as a constellation of spring‐fed forks in Copper Basin, tumbles through sagebrush flats, and then slips underground before resurfacing near Arco. The jewel of the system is the 7-mile tailwater below Mackay Reservoir, a cold, fertile reach that grows rainbow and brown trout well over 20 inches. With dependable year-round water temps, miles of public access on Bureau of Land Management (BLM) parcels, and wild back-country headwaters, the Big Lost offers something for first-time dry-fly anglers and technical nymph specialists alike.
Anglers divide the drainage into three zones. **Upper Big Lost (Copper Basin)** weaves through sprawling meadows where native Yellowstone-cutthroat and brook trout rise eagerly to attractor dries. **Tailwater (Mackay Dam to Leslie)** is a classic spring-creek-like ribbon: slow slicks, deep buckets, and nutrient-rich weed beds harboring trophy rainbows. **Lower Big Lost (Leslie to Arco)** braids through irrigation canals and cottonwood bottoms; flows fluctuate with farm demand, but when water is up, streamer fishing for browns can be excellent. Each stretch fishes best at different times, rewarding exploration.
Wild rainbow trout dominate the tailwater, averaging 14–18 inches with 22-inch brutes caught every season. Brown trout numbers are lower but size potential is impressive. In the high country, Yellowstone-cutthroat, brook trout, and mountain whitefish thrive in cool tributaries like Wildhorse and Star Hope creeks. The lower river supports smaller rainbows, brown trout, and occasionally white suckers when flows persist through summer.
Abundant midges and baetis keep trout feeding through winter and early spring. May and June see explosive crane-fly and caddis emergences followed by Salmonfly and golden-stone hatches in higher flows. Pale Morning Duns, yellow sallies, and trico clouds fuel steady surface action midsummer, while late-summer hoppers, beetles, and flying ants entice bank-tight strikes. Autumn brings aggressive streamer chases as rainbows fatten before lean winter months.
A 9-foot 5-weight with floating line is the workhorse for dries and nymphs; carry a 6-weight or short trout-Spey with sink-tip to swing streamers in fall. Tailwater trout demand long 12-ft leaders tapered to 5X-6X and tiny #20-22 midge pupa under yarn indicators or euro-nymph rigs. In Copper Basin, high-vis attractor dries (#10-14) and dry-dropper setups excel. During mid-summer irrigation pulses, stripping olive sculpin or articulated leech patterns along grass edges moves trophy browns.
U.S. Highway 93 pull-outs below Mackay Dam provide quick access to famous runs like the “Tubes,” “Narrows,” and “Miracle Mile.” In Copper Basin, day-use areas at Star Hope Campground and the Forks Campground offer meadow riffles ideal for families. Roadside pull-outs along Antelope and Wildhorse creeks supply lightly pressured pocket water. Lower-river float anglers put in at Leslie or Moore bridges when irrigation releases allow adequate depth.
From Mackay, follow Trail Creek Road (Highway 508) for multiple gated turnouts along the tailwater—respect posted private land signs. BLM and Idaho Department of Lands parcels are well-signed and open to the public. Upstream, Copper Basin Road loops through the basin with campsites and cattle guards; most meadow stretches are on public land. Cell service is sparse—carry paper maps or download offline layers, and check spring road conditions before driving high passes.
Tailwater success hinges on light 5X-6X tippet, size 18–22 zebra midges, baetis emergers, and tan scuds. Pack #12 crane-fly adults, #16 caddis, and #10 golden-stone nymphs for early summer. Terrestrial box: hoppers (#8-10), cinnamon ants (#14), and beetles (#12). Streamer box: olive sculpzillas, black leeches, and white articulated minnows. Breathable waders and felt or studded rubber soles grip slick lava rock; in Copper Basin wet-wading shoes suffice midsummer. Polarized sunglasses and high-SPF sunscreen are musts under intense alpine sun.
The Big Lost is chronically de-watered for irrigation, and late-summer flows can drop below 30 cfs—IDFG conducts annual fish rescues to move stranded trout into deeper pools. Trout Unlimited’s Hemingway Chapter partners with ranchers to install fish-friendly diversions and timed withdrawals. Anglers can help by minimizing fish handling during low-flow afternoons, using thermometers, and supporting water-leasing programs that keep instream flows viable for native trout.
Copper Basin sits above 7,500 feet—afternoon thunderstorms develop quickly; pack layers and a rain shell. Spring runoff can double flows overnight, and ice shelves form along the tailwater in winter. Rattlesnakes inhabit sage benches below Mackay; watch footing when bushwhacking to banks. Remote stretches have no cell coverage—carry a satellite communicator, spare tire, and extra fuel. In irrigation season, sudden release spikes can make wading the tailwater hazardous—check Mackay Reservoir outflow gauges each morning.
**Lost River Outfitters** (Ketchum) offers daily Big Lost reports, guided trips, and a full fly selection. **Silver Creek Outfitters** in Ketchum stocks technical tailwater patterns, rents rods, and arranges Copper Basin pack-in trips. **Sun Valley Outfitters** provides guide services, shuttle coordination, and up-to-date flow intel. All three shops welcome beginners and can secure permits for wilderness horse-pack adventures into the headwaters.
The **Hemingway Chapter of Trout Unlimited** leads riparian fencing, fish-friendly irrigation diversion upgrades, and temperature monitoring on the Big Lost and its tributaries. **Wood River Land Trust** collaborates with ranchers on voluntary water-saving measures that leave more late-season flow for trout. Regionally, the **Idaho Conservation League** advocates for balanced water policy safeguarding aquifer recharge and instream habitat. Donations, volunteer days, or citizen-science data submissions directly bolster Big Lost trout health.
The Big Lost River and its tributaries fall under Idaho’s Upper Snake Region rules. From December 1 until the Friday before Memorial Day weekend, all trout (except brook trout) and whitefish are catch-and-release only. Beginning the Saturday of Memorial Day weekend through November 30, anglers may keep up to six trout per day, while mountain whitefish remain catch-and-release year-round to protect the unique native population. Bull trout must be released immediately if encountered, and brook trout may be harvested under the general 25-fish brook-trout limit. Always consult the current Idaho Fish & Game Seasons & Rules booklet and verify any emergency closures before fishing.