Fly Fishing the Little Lost River, Idaho: Desert Trout

A River That Ends in the Ground

The Little Lost River runs about 49 miles through Butte County in east-central Idaho. It begins where Summit Creek and Sawmill Creek meet at roughly 6,100 feet, then flows southeast down a long, dry valley. The Lost River Range walls that valley on the west and the Lemhi Range on the east.

The river never reaches another river. Near the town of Howe, at about 4,800 feet, it sinks into fractured lava rock and joins the Snake River Plain Aquifer. That is why Idahoans call it a “sinks” stream. The whole drainage covers about 963 square miles, and Wet Creek is its largest tributary.

You may see the source placed in the Beaverhead Mountains. That is a different range. The Beaverheads sit a full valley farther east, beyond the Lemhi Range.

Idaho Fish and Game lists the Little Lost as a Recommended Fishing Water. Its planner page notes that the remote setting keeps crowds away most of the year, so the fish see little pressure. That matches what you find: a small, clear trout stream with plenty of empty water. For more streams like this, see our Idaho secret waters guide.

Regulations (Checked September 2026)

The Little Lost River and its tributaries are a Special Rule Water in Idaho’s Upper Snake Region. The rules differ from the general regional rules, so read this table before you keep a fish.

The general Upper Snake trout limit is six, so this is a reduced-limit water. The current rule book covers 2025 through 2027. Check the IDFG seasons-and-rules page before you go, because special rules can change.

Getting There: Howe, Sawmill Canyon Road, and Mill Creek

Howe is the gateway town. From Howe, the Little Lost/Pahsimeroi Road runs 37 miles north up the valley to Sawmill Canyon Road, which is Forest Road 101. That road climbs into the headwaters and is the main access to the upper river.

Forest Service directions:

• From Arco: US 20/26 south 8 miles. Left on State Highway 22/33, 16 miles to Howe. Straight through Howe onto the Little Lost/Pahsimeroi Road, 37 miles. Right on Sawmill Canyon Road.
• From Mackay: US 93 south 7 miles. Left on Pass Creek Road, 28 miles to the Little Lost/Pahsimeroi Road. Left for 0.2 miles, then right on Sawmill Canyon Road.

Mill Creek Trailhead and Campground is the camping option in the headwaters. Turn right off Sawmill Canyon Road onto Mill Creek Road and follow it to the end.

Idaho Fish and Game lists no boat ramp, toilet, or campground of its own on the river. Howe has almost no services. Fuel up and buy food in Arco or Mackay before you head in. Cell coverage fades in the valley, so download your maps ahead of time.

Flows by Month: What the USGS Gauge Says

The USGS gauge “Little Lost River below Wet Creek near Howe,” station 13118700, has measured this river since 1958. Its long-term monthly averages, in cubic feet per second (cfs), tell you when to come.

Source: USGS monthly statistics, 1958 to 2024.

Snowmelt peaks in June at roughly three and a half times the September flow. July drops by half. From August through October the river settles near 50 cfs, which is easy wading on a stream this size. Winter runs near 20 cfs under ice and snow.

On the morning of 6 September 2026 the gauge read 36 cfs. That is well below the September average, so 2026 is a low-water year. Expect skinny riffles and nervous fish this fall. Check the live reading on the USGS gauge page before you drive out.

The season is open year-round, but the road is another matter. The Forest Service lists Sawmill Canyon as a snowmobile area in winter, and the Mill Creek campground does not open until May 20. Plan on late June through October for the headwaters.

The Fish: Wild Rainbows, Brook Trout, and Protected Bull Trout

Idaho Fish and Game names three game fish here: rainbow trout, brook trout, and bull trout. Department surveys found rainbows in 2021, brook trout in 2016, bull trout in 2009, and sculpin in 2006.

The rainbows are stream-born now. IDFG stocking records show rainbow plants from 1968 through 2008, including 70,000 fingerlings in May 1984. The last three plants, in summer 2008, were about 170 catchable fish each. Nothing has been stocked since, so every rainbow you catch comes from fish that have reproduced on their own for close to two decades.

Brook trout are not native to Idaho and were stocked long ago. Salmon-Challis National Forest fisheries biologist Bart Gamett told Trout Unlimited that mountain whitefish were documented here in the early 1900s but no longer turn up. The occasional cutthroat, he said, is almost certainly of hatchery descent.

Bull trout are the reason this river matters to biologists. The Endangered Species Act protects them as a threatened species. The 2015 federal recovery plan names the Little Lost River as one of 22 bull trout core areas in the Upper Snake Recovery Unit.

It is an isolated population. No surface water links this valley to the rest of the species’ range, and how the fish got here is still debated.

Gamett surveyed the river system in the late 1990s and described two theories. One says the fish are native and were cut off from the Pahsimeroi River drainage by lava flows and geologic change. The other says early settlers moved them here through irrigation canals or in buckets. He called the settler theory more likely, but said real uncertainty remains.

Whatever the origin, the rule is simple. Idaho allows no bull trout harvest anywhere in the state, and on the Little Lost the regulation says catch-and-release. Don’t fish for them on purpose. If one takes your fly, keep it in the water, back the hook out, and let it go. Skip the photo.

Telling a bull trout from a brook trout matters here. Bull trout show pale spots on an olive body and no black marks on the dorsal fin. Brook trout carry wavy, worm-like markings on the back and a black-and-white edge on the lower fins. The shorthand anglers use is “no black, put it back.”

How to Fish It

This is small water, and it fishes like small water. A 3- to 5-weight rod with a floating line covers everything. Fish a buoyant attractor dry through summer, and hang a small beadhead nymph under it when fish won’t rise.

From late July into September, hoppers and ants along grassy banks pull the best fish out. During June runoff, go subsurface with weighted nymphs or a small streamer swung through the softer edges.

At 36 cfs the fish have nowhere to hide, and they know it. Approach from downstream, stay low, and lengthen your leader. One sloppy cast empties a pool.

Pinch your barbs. It costs you almost nothing on rainbows this size, and it means a hooked bull trout goes back in seconds.

Pair the trip with two very different Idaho waters: the Snake River carp flats for sight fishing on a bigger scale, or Jordan Creek for another quiet small stream.

Frequently Asked Questions

Where does the Little Lost River go?

Nowhere on the surface. The river flows about 49 miles southeast from the meeting of Summit Creek and Sawmill Creek to the town of Howe. There, at about 4,800 feet, it sinks into fractured lava rock and joins the Snake River Plain Aquifer.

What fish live in the Little Lost River?

Idaho Fish and Game lists rainbow trout, brook trout, and bull trout, and surveys have also found sculpin. Rainbows and brook trout are legal to keep under the 2-trout limit. Bull trout are federally threatened and must be released.

What are the fishing regulations on the Little Lost River?

The Little Lost River and its tributaries are open year-round with a trout limit of 2, all species combined. Bull trout are catch-and-release only. These are Upper Snake Region special rules, current through 2027. Check the IDFG rule book before your trip.

When is the best time to fish the Little Lost River?

Late July through October. USGS records show runoff peaks in June at about 178 cfs, drops by half in July, and settles near 50 cfs from August through October. The Forest Service campground at Mill Creek opens May 20, and the canyon road is snow country in winter.

Sources and Check Dates

• Idaho Fish and Game, Fishing Planner: Little Lost River (species, survey years, stocking records). Checked 6 September 2026.
• Idaho Fish and Game, 2025–2027 Seasons and Rules, Upper Snake Region, Special Rule Waters. Checked 6 September 2026.
• USGS gauge 13118700, Little Lost River below Wet Creek near Howe: monthly statistics 1958–2024 and the reading of 6 September 2026.
• USDA Forest Service, Salmon-Challis National Forest: Mill Creek Trailhead & Campground; Sawmill Canyon Area. Checked 6 September 2026.
• US Fish and Wildlife Service, Recovery Plan for the Coterminous United States Population of Bull Trout (2015), Upper Snake Recovery Unit.
• Trout Unlimited, “Wild: Little Lost River bull trout” (2017), reporting Bart Gamett, Salmon-Challis National Forest.
• Wikipedia, “Little Lost River” (length, source, elevations, drainage area).