A river that ends in the desert
The Big Lost earns its name. Its USGS gauges run from the headwaters to a station above the Big Lost River Sinks near Howe, and on the morning of September 24, 2026 they told the whole story: about 265 cfs below Mackay Reservoir, about 190 near Leslie, and then zero at the gauge below the Moore Diversion, zero at Arco and zero near Arco. The water that leaves Mackay Dam is spoken for by the ditches of the valley, and the river below them is a dry bed for much of the year.
For an angler that narrows the map. The fishing is in two places: the tailwater below Mackay Reservoir, where the gauge on this page sits, and the headwaters and tributaries up in Copper Basin on the east side of the Pioneer Mountains. The lower river toward Arco is not a fishery you plan around.
Below Mackay Dam
Lost River Outfitters, the Ketchum shop that took its name from this river, calls the tailwater below Mackay Reservoir home to some of the largest and healthiest rainbows in the area. It runs under the 12,000-foot peaks of the Lost River Range through cottonwood and aspen, with hatches of stoneflies, caddis, pale morning duns, blue-winged olives, crane flies and tricos. The shop also leases a two-mile private beat just below the dam, which carries a rod fee and can be fished only with one of its guides.
The gauge record, going back to 1903, shows an irrigation river. Median monthly flow is about 105 to 125 cfs from January through April, then climbs to about 400 in May, 840 in June and 620 in July, before dropping to about 395 in August, 210 in September, 150 in October and under 90 in November and December. No shop publishes a wading band, so the 100 to 250 cfs range on this page is drawn from that record and from the shop's latest posted report, which said that below Mackay, at 452 cfs, wading was extremely dangerous, fish were hard to find, and heavy, deep indicator rigs were the only real option. September and October are when the tailwater comes back within reach.
Two seasons in the rulebook
Idaho Fish and Game's 2025 to 2027 rules cover the Big Lost River and all its tributaries as one water in the Upper Snake Region. From December 1 through the Friday before Memorial Day weekend, trout are catch-and-release, except brook trout. From the Saturday of Memorial Day weekend through November 30, the trout limit is six. Whitefish are catch-and-release in both seasons, an exception to the region's general whitefish limit of 25 that the rules call out by name.
The Big Lost is also where you want to be careful with identification. Lost River Outfitters lists rainbows, cutthroat, brook trout and occasional grayling in the upper river, and bull trout, where they occur anywhere in Idaho, are catch-and-release under the general rules. If you move on to Mackay Reservoir or another nearby water, look up its own entry rather than assuming the river's rules carry over.
Up in Copper Basin
The river begins above 10,000 feet on the east side of the Pioneer Mountains, and the upper Big Lost is a web of meadow streams and tributaries through Copper Basin. Lost River Outfitters describes frequent hatches and consistent dry-fly fishing all season, with rainbows, cutthroat, brook trout and the occasional grayling, and it is common to see antelope, elk, moose or black bears on the drive in. Sage grouse gather in the basin in summer and early fall.
The same posted report for the upper river had it running about 299 cfs with great clarity, still pushier than its other local waters, with bugs hatching through the day and nymphs taking the picky cutthroat. Its suggested patterns were terrestrials, PMDs, yellow sallies, Rubber Legs, Zebra Midges, Pheasant Tails and Lightning Bugs.
The Big Lost hatch chart, month by month
Silver Creek Outfitters, also in Ketchum, publishes a Big Lost chart that lines up with Lost River Outfitters' list. Assorted stoneflies, sizes 6 to 14, come in June and early July; crane flies, sizes 10 to 14, from June through August; PMDs, 16 to 18, from mid-June into August. Tricos, 20 to 24, run from mid-July through September; baetis, 18 to 22, from mid-August through October; and hoppers, 4 to 10, from August into September. From December through March it is midges, 20 to 24.
Because the tailwater usually runs high through June and July, the stonefly and PMD windows are often best fished in Copper Basin, while the trico, baetis and hopper months line up with the tailwater's fall drop.