Fly Fishing the Little Lost River, Idaho: Desert Valley Trout

Why the Little Lost River Is Worth the Trip

If you want a taste of authentic Idaho fly fishing without the elbow-to-elbow crowds, fly fishing the Little Lost River, Idaho delivers it. This small desert-valley stream sits a short drive from Idaho Falls, yet most anglers pass it by entirely.

That’s the appeal. You get canyon rainbows with vivid color, scrappy brook trout, and a rare shot at a native char found almost nowhere else in the region. The river stays quiet because it doesn’t look like much on a map — a thin blue line that seems to end in the middle of nowhere. It does. That’s part of the story.

For a broader look at water like this across the state, see this Idaho secret waters guide.

A River That Never Reaches the Sea

Most rivers have an obvious ending: a bigger river, a lake, an ocean. The Little Lost breaks that pattern entirely.

It starts as melting snow and mountain springs draining off the Beaverhead Mountains. From there, it runs south and spreads out across the Big Desert, a flat, sagebrush-covered stretch of ground near the Idaho National Laboratory. Once it hits that desert floor, the water simply disappears. The ground there sits on old lava flows, riddled with cracks and voids, and it swallows the current whole.

Hydrologists believe that water doesn’t vanish for good. It travels underground for miles and is thought to feed the spring complex near Hagerman, far to the southwest. A handful of other streams in the region, including Birch Creek, behave the same way — geologists call them sink streams, since they sink rather than flow onward on the surface.

Knowing this changes how you read the river. The Little Lost isn’t broken or unfinished — it’s just following a different plumbing system than the rivers you learned about in school.

Where It Is and How to Get There

The Little Lost begins in the Beaverhead Mountains and runs south to the small settlement of Howe, Idaho, roughly a couple of hours from Idaho Falls by car.

Quick reference for planning your trip:

Sawmill Canyon is the easiest way in, and it’s a good base if you want to camp streamside. Mill Creek campground sits right in the canyon and rarely fills up, even on a holiday weekend. Bring an Idaho road atlas or offline maps — cell coverage thins out fast once you leave the highway.

Above the campground, a dirt road follows the canyon until it simply stops. From that point, the only way forward is on foot.

The Fish You’ll Find (and the One You Must Release)

Three trout species share this water, and each behaves a little differently depending on where you are in the drainage.

Rainbow trout dominate the canyon stretch near Sawmill Canyon. They run bright, often carrying a deep red-orange band down each flank, and they attack streamers and dry flies with real aggression. In the canyon, fifteen-inch fish are a realistic target on a good day. Farther downstream, where the river spreads into open, sage-lined valley, the average size climbs and trophy-class rainbows become possible.

Brook trout share the canyon water with the rainbows. They’re smaller on average but every bit as willing to chase a fly, and a heavy brookie here still puts up a real fight.

Bull trout are the river’s rarest resident, holding mainly in the upper end of Sawmill Canyon. This population is isolated — no surface water joins this valley to the Snake River drainage where bull trout are typically native, and biologists still debate how they got here. One theory points to an ancient waterway that once linked this valley with the Pahsimeroi River drainage before a geologic shift severed it in this seismically active corner of Idaho. Another theory credits early settlers, who may have moved fish here from a nearby native population decades ago.

However they arrived, bull trout on the Little Lost carry full federal protection under the Endangered Species Act. Intentionally fishing for them is illegal, and because they’re aggressive predators of other fish, one may strike your dry fly or streamer even when you’re targeting rainbows. If that happens, keep the fish in the water, remove the hook quickly, and let it go without lifting it out for a photo. Treat every bull trout as an accidental catch, not a goal.

Best Flies for the Little Lost

These trout haven’t seen enough fishing pressure to get fussy, so pattern choice matters less than matching the season. Here’s what works through the year:

• Early season (runoff): Rio Grande King wet fly, Stayner Ducktail — swing these through discolored, high water
• Early-to-mid summer: Stimulator, Adams, Royal Coachman — solid attractor dries once flows settle
• Midsummer through fall: foam hoppers, foam beetles, foam ants — go with smaller sizes as you move upstream into the headwaters

Terrestrial insects are abundant along the whole river by late summer, so a hopper-beetle-ant rotation can carry you through the hottest stretch of the season.

Gear That Fits This Water

Match your rod to where you’re fishing rather than bringing one setup for the whole river.

• Lower river and canyon water, streamers and hoppers: a 5-weight rod with floating line handles the wind and the bigger flies without feeling like overkill.
• Upper river and headwaters: drop down to a 3- or 4-weight with floating line — the water narrows, and a lighter rod presents small dries more delicately.
• Tight backcountry water: consider a tenkara rod. These fixed-line Japanese rods skip the reel entirely, which makes short, precise casts easier in brushy, narrow water. The Little Lost’s small headwater sections are close to ideal for trying the method for the first time.

Wading is straightforward in most stretches, but the upper canyon requires some scrambling around downed timber left from an old wildfire, so wear boots that can handle uneven footing.

When to Go

Timing on the Little Lost tracks the seasons closely, since it’s fed largely by snowmelt.

Spring runoff pushes the river high and cloudy, which is exactly when swung wet flies produce best. As flows drop through early summer, switch over to attractor dry flies fished on top. By midsummer, terrestrial patterns take over as grasshoppers and beetles become the dominant food source trout are watching for.

One catch: by late summer, the upper river — especially above Sawmill Canyon — can shrink to barely a trickle. Plan headwater trips for June or July if you want more water to work with, and save the driest months for the lower canyon and valley stretches, where flow holds up better.

For other regional water worth pairing with this trip, check out the Snake River carp flats for a completely different kind of fly-fishing challenge, or Jordan Creek for another quiet southern Idaho small stream.

Frequently Asked Questions

Where does the Little Lost River go?

It never reaches an ocean. The Little Lost flows out of the Beaverhead Mountains, crosses the Big Desert near the Idaho National Laboratory, and vanishes into porous, lava-based ground. That underground water is believed to resurface far to the southwest, near the springs around Hagerman.

What fish live in the Little Lost River?

Introduced rainbow and brook trout make up most of the catch, with rainbows in Sawmill Canyon showing a bold red stripe. The river also holds a rare native population of bull trout. Bull trout are federally protected — never target them, and release any you hook without lifting them from the water.

When should you fish the Little Lost?

Fish wet flies during spring runoff, switch to attractor dry flies once flows drop in early-to-mid summer, and move to foam terrestrials — hoppers, beetles, ants — from midsummer into fall. By late summer the upper river thins to a trickle, so expect the best flows earlier in the season.