Current Flows of the South Fork of the Snake River

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What is the South Fork of the Snake River?

If you’re a fly angler, the South Fork of the Snake River likely needs little introduction. Flowing from Palisades Reservoir in eastern Idaho, this 66-mile stretch of river winds through high mountain valleys, steep canyons, and broad flood plains before joining the Henry’s Fork near Menan to form the main stem of the Snake River.

This section of the river, known simply as the South Fork, is regarded as one of the finest trout fisheries in the country. With up to 4,200 fish per mile (according to 2017 data at Conant), it offers anglers an unmatched Blue Ribbon experience. Native Yellowstone cutthroat trout dominate here, along with brown and rainbow trout. Anglers are encouraged to harvest rainbows, which can outcompete native cutthroat, while a slot limit requires all fish between 8 and 16 inches to be released to protect key breeders.

The river is best fished from July through October, when dry fly action is at its peak. Early July features prolific stonefly hatches—especially the massive salmon fly, which can reach three inches long—bringing trout to the surface in a feeding frenzy.

From Palisades Dam, the South Fork begins in a narrow canyon channel and gradually transitions into a dynamic floodplain with braided channels and island complexes. Scenic highlights include Fall Creek Falls near Swan Valley Bridge and the rugged canyon below the Conant boat launch. Downstream near Heise Hot Springs, the cliffs give way to flatter, more open terrain.

The area surrounding the river is just as spectacular as the water itself. The corridor is home to the largest riparian cottonwood gallery forest in the western U.S. and is among the most diverse ecosystems in Idaho. It supports 126 bird species—including 21 species of raptors—and a wide variety of wildlife such as moose, elk, mountain lions, black bears, bobcats, beavers, and more. The South Fork is recognized as a National Important Bird Area.

Recreation opportunities abound beyond fishing. The South Fork area includes access to Yellowstone and Grand Teton National Parks, Island Park, Targhee National Forest, and Henrys Lake State Park. Hiking, camping, boating, and wildlife viewing draw over 300,000 visitors annually. Trails like the Canyon Rim Trail and others in the Swan Valley region offer spectacular views and access for mountain biking and off-highway vehicles.

Land along the South Fork is managed by a mix of agencies and private owners, including the Bureau of Land Management, the Forest Service, and the Idaho Department of Fish and Game. Several developed boat access points—such as Byington, Conant, Lorenzo, and Menan—facilitate recreational use. The river is also eligible for inclusion in the National Wild and Scenic Rivers System, reflecting its environmental and recreational value.

SOUTH FORK OF THE SNAKE FISH SPECIES

Three major salmonid species inhabit the 64 miles of river: Yellowstone cutthroat trout, brown trout, and rainbow trout. With Yellowstone cutthroat and rainbow trout sharing the water, hybrid cut-bows are also present—but we’ll come back to those. First, let’s focus on the main attraction.

Yellowstone cutthroats are aggressive surface feeders, making the South Fork one of the world’s premier dry fly fisheries. As you’ll see in the hatch calendar, there are few times when dry fly fishing isn’t an option along this stretch. If that doesn’t inspire a trip, we’re not sure what will.

Brown trout are found throughout the river, with especially strong populations in the lower sections. Fish exceeding 20 inches are not uncommon, particularly if you're fishing streamers along cut banks during high flows.

Rainbow trout, meanwhile, have sparked controversy. Idaho Fish and Game has resorted to shocking and removing rainbows and lifted bag limits entirely to reduce their numbers. Their goal is to restore native cutthroat populations, as rainbows both interbreed with and prey on cutthroat redds.

This has ignited heated debate. Many guides rely on rainbows to keep clients happy and argue that removal efforts harm the broader fishery. Others point out that none of these species are truly native anymore. Nonetheless, the rainbows remain a central point of tension, with ongoing political and ecological debates. While there’s no limit on harvesting them, most guides still value and protect these fish as essential to their business.

SOUTH FORK OF THE SNAKE HATCHES

There are many excellent times to fish dry flies on the South Fork, with the prime season running from the third week of June through October. The first two weeks of June typically see the reservoir being flushed, resulting in high flows and poor fishing conditions. Outside of the main season are the shoulder months, which can still offer productive fishing—May, for instance, has yielded good results for the Moonshine team.

Thanks to Brett Novik, a longtime guide on the South Fork, for sharing the following hatch breakdown:

March through June

Best suited for nymphing, though hardy anglers may encounter PMD hatches during stormy weather.

Late June

Dry fly action kicks off with the salmon fly hatch in the fourth week of June. This is the time to target big fish with big dries. Yellow Sallies and PMDs follow, along with a small golden stonefly hatch that continues into early August.

July

Yellow Sallies and PMDs persist. Around the third week, watch for a short-lived green drake hatch—an exciting recent addition to the river’s insect lineup.

August

PMDs remain active. Starting mid-month, a unique stonefly hatch known as the “mutants” emerges, running into early September. These insects can’t fly and skate across the surface, providing thrilling topwater action from dawn through late morning.

September

PMDs continue, joined by mahoganies. Blue-winged olives begin hatching in late September and continue through mid-October.

Late Fall to Early Winter

Streamer season begins. Large browns move into the shallows, prowling cutbanks and flats. This is prime time for targeting trophy fish with big streamers.

Nymphing remains a reliable year-round strategy, especially in winter and early spring. For current conditions during shoulder seasons, local fly shops are your best resource.

Sections of the South Fork of the Snake River for Fishing

The Upper Section

At the base of Palisades Dam, the Snake River transforms into the legendary South Fork, beginning a 16-mile journey through the idyllic alpine landscapes of Irwin and Swan Valley—one of the most iconic river stretches in the American West. Tight and fast in some areas, wide and sweeping in others, it’s a river of unmatched character. Anglers can hook big tailwater browns, rainbows, and cutthroats using nymph rigs and streamers. Fed by cold, oxygen-rich water from the depths of Palisades Reservoir, the river stays chilly even on the hottest summer days and teems with food like mysis shrimp.

Below the confluence with Palisades Creek, the river narrows, offering a more intimate float ideal for casting dry flies along the banks. During the salmonfly hatch in late June and early July, and again in August and September when terrestrials are abundant, trout rise aggressively to big dries.

Near Irwin, the river opens into a braided network of riffles, runs, and sloughs. This section features Fall Creek Falls and offers excellent walk-and-wade fishing—some of the finest for wild trout in the West.

As the river nears Spring Creek Bridge, it tightens again into a 2-mile stretch of prime trophy trout water. From the bridge to the Conant takeout, this section offers unforgettable days on the river, with fishing conditions that vary beautifully with the seasons and hatches.

The Canyon

Just downstream of the Conant boat ramp lies the storied canyon stretch of the South Fork of the Snake River. For 22 miles, the river winds through basalt cliffs and a remote, lush cottonwood forest — a wild float through untamed country rich with trophy trout.

This canyon was carved through ancient lava flows left behind by the last major eruption of the Yellowstone supervolcano. The river's path reveals this dramatic volcanic history, though most anglers are too focused on the fishing to fully take in the geologic scenery.

Instead, they’re navigating hidden side channels, exploring quiet sloughs, and walking riffles in pursuit of the next memorable trout. Browns, rainbows, cutthroats, hybrids, and mountain whitefish are abundant, thriving in what is considered some of the best trout habitat in the West.

The canyon offers reliable protection from eastern Idaho’s notorious winds, and its waters are a dry-fly angler’s dream during both the early-summer stonefly hatch and the late-summer terrestrial season. Big browns, cutthroats, and rainbows aggressively rise to surface bugs, making this stretch a magnet for serious fly fishers.

Renowned for its consistent action and healthy wild fish, the canyon stretch of the South Fork is a must-fish destination — an unforgettable experience every avid angler should have at least once.

Lower Canyon

About an hour’s drive from the Conant boat ramp, through the heart of Idaho’s productive barley and grain fields—some of which contribute to the world’s best beer—is the Cottonwood/Fulmer boat ramp. This marks the beginning of the float through the lower canyon stretch of the South Fork of the Snake River.

Much like the upper canyon, this stretch features dramatic cliff walls and lush cottonwood forests. But as the river flows downstream, the scenery shifts to arid bluffs and high desert hills, gradually working its way toward a confluence with the famed Henry’s Fork.

Fishing in the lower canyon mirrors that of the upper stretch, with big, wild trout rising to dry flies starting in June and continuing through late summer and hopper season. As the river warms with distance from the cold-water release at Palisades Dam, brown trout become more prevalent—and often larger.

Dry-fly enthusiasts will find excellent opportunities here. Green Drakes emerge in June and into July, offering topwater action that rivals the celebrated salmon fly hatch beginning in mid-to-late June. These big mayflies get the trout tuned in just in time for the stoneflies.

But the lower canyon isn’t just for dry-fly purists. Streamer anglers can find thrilling action swinging flies through deep runs, under submerged logs, and along grassy banks where trophy browns often lurk. Overcast days bring out the bruisers, making it prime time for those chasing their fish of a lifetime.

For anglers with limited time, the lower canyon is especially appealing due to the short distances between put-ins and takeouts.

Byington to Lorenzo

The stretch of the South Fork of the Snake River between the Byington put-in and the Lorenzo takeout is affectionately known to local anglers as Brown Town—and for good reason. This reach, marking the transition from the South Fork’s canyon section to its lower river, is prime water for those targeting big brown trout.

As the river flows through agricultural lands, it begins to lose a significant portion of its volume to irrigation diversions—up to half by the time it reaches Lorenzo. These changes continually reshape the river, making each float a unique experience. Anglers can explore different side channels with each trip, often never rowing the same route twice.

Though it runs through a more developed part of eastern Idaho, Brown Town feels wild. Towering cottonwoods shade the banks, helping keep the water cool in summer and providing a surprisingly secluded atmosphere. Grass-lined banks become hotspots during terrestrial season (late July through early October), and summer-long bug activity—including a golden stonefly hatch early in the season—keeps fish feeding on the surface.

While big browns are the main draw, rainbows, cutthroats, and whitefish also inhabit this stretch. Trout numbers may be lower than in cooler, upper reaches of the South Fork, but the fish here—especially the browns—tend to be larger.

Anglers can choose a full-day float from Byington to Lorenzo or opt for shorter floats, such as Byington to Heise or Twin Bridges. While many anglers frequent other sections of the South Fork, those in search of trophy browns often find their way to Brown Town—home to some of the river’s most impressive fish.

Lorenzo to Menan

The lowest stretch of the South Fork is commonly known as “Section 5,” though local anglers often refer to it as the Lorenzo-to-Menan float, after the two public access points. This section runs from the Lorenzo boat ramp, just west of the U.S. Highway 20 bridge near Rigby, Idaho, to the takeout just above the confluence with the Henry’s Fork near Menan.

Though often overlooked, this float can produce impressive brown trout—especially from mid-June through July during the salmon fly hatch, and again from late summer into October when grasshoppers are abundant. Anglers targeting large browns should give this section serious consideration.

While rainbows, cutthroats, and whitefish are still present, warmer water temperatures favor browns. Fish numbers are lower here than in other reaches, but those that remain tend to be larger. Despite the warmth, this stretch stays cooler than the Henry’s Fork near their confluence, drawing in big browns and rainbows from both the Henry’s Fork and the mainstem Snake River as summer progresses.