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Fly Fishing the South Fork Boise River

Heavily spotted and bright-sided rainbows that feed consistently on abundant mayfly, stonefly, and caddis hatches.

Fly Fishing the South Fork Boise River
Photo: Greg Thomas 

You can't really call southern Idaho's South Fork Boise River a secret because local anglers pound it. However, it has always remained a bit below the national angling radar, especially when compared to the Gem State's glory waters Silver Creek, the Henry's Fork, and the South Fork Snake.

Certainly the South Fork Boise, which lies about 70 miles east of Boise, is one of the West's most wonderful wild-trout fisheries, offering heavily spotted and bright-sided rainbows that feed consistently on abundant mayfly, stonefly, and caddis hatches. The river flows through a remote arid canyon, which abounds with wildlife, including mule deer, black bears, chukar partridge, ruffed grouse, and rattlesnakes. You won't see second homes and no-trespassing signs on the banks of this stream for most of its length, the South Fork flows through U.S. National Forest or Bureau of Land Management (BLM) land.

In recent years the South Fork has been especially good for large wild rainbows. Prior to 2004 it was a minor miracle to take a 'bow larger than 20 inches on the South Fork. During the mid-1990s, over the course of at least 50 angling days on the South Fork, I tallied only two fish over 20 inches. (One was a 22-inch tanker.)

In recent years, for reasons that biologists don't really understand, anglers are catching more big trout. Frequent reports of fish over 20 inches and my own recent experiences tell me there are more big trout on the Boise right now than at any time in recent memory. Because South Fork anglers release nearly 90 percent of their catch, those fish are surviving. Another attractive aspect of the South Fork is its winter catch-and-release season (December through March 31). Anglers can fish here nearly all year, which is not the case on many of Idaho's other top trout waters.

The South Fork is a "governor's river," meaning that commercial guiding is not allowed. I'm not against commercial interests on some rivers, but it's nice to see anglers following their own paths to angling heaven, and in general it means less competition from other anglers.

Thus the South Fork, due to its remote location, its wild trout, and its scenic attributes including that lack of private residences is everything that Silver Creek, the Big Wood, the Ruby, and the North Platte are not. You can fish without pressure; pitch a tent or park a trailer without the threat of flashlights being shined in your eyes around midnight; build a fire (unless there are seasonal restrictions); or uncap a bottle and howl at the moon as loud as you like. In this age of population expansion and land-access restrictions in the West, that's a rare and special experience.

Anderson Ranch Dam

The South Fork begins in south-central Idaho's Smoky Mountains north of Pine. Just a small freestone stream at its headwaters, it holds fair numbers of rainbow, bull, and brook trout.

The South Fork's most desirable and heavily fished water begins at the base of Anderson Ranch Dam and extends downstream about 26 miles to Neal Bridge.

Below Neal Bridge, the river is managed under general regulations. The river loses its identity a few miles below Neal Bridge, where it becomes Arrowrock Reservoir.

Between Anderson Ranch Dam and Danskin Bridge, about a 10-mile section, a dirt road parallels the river and provides almost unlimited wade access. This is where most of the South Fork fishing occurs and, at times, it can be overwhelmed by anglers. Nearby Boise used to be a small town but is recently prosperous, and has grown to 500,000 people. Angling competition on the South Fork will grow as the population continues to increase.

If you don't like company, check the water levels, launch a raft at Danskin Bridge, and float the roadless section to Neal Bridge, which offers about 16 miles of underfished water with big trout. One reason it is less pressured is the skill required to navigate a raft through this stretch. The lower half of the float is fast-paced and contains named Class II and III rapids, including Raspberry and the Devil's Hole. Either can place a novice oarsman—and everyone in his boat in the drink.

If you float the roadless stretch, take your time on the upper portion, stopping to fish the water thoroughly. You may choose to camp overnight on the river. Fish the fast water the next day, casting streamers or big drys to the banks.


You can also float the upper river, and there are several prime options between the dam and Danskin Bridge. However, flows may climb to 3,000 cubic feet per second (cfs) or more during May, June, and July. Prime boating flows range between 1,400 and 1,800 cfs. Only experienced oarsmen should take the oars, no matter what the flow registers.

Trip Timing

The river runs highest and fastest from Memorial Day weekend through July. Wade fishing can be a challenge, and spiked boots and a wading staff are advisable. Fortunately, the South Fork is heavily braided, and the many shallow side channels provide decent wade-fishing in high flows when trout move into them.

During high spring flows, you should not wade across the river attempting to reach fish Simple upstream casts along the banks are highly effective. Wade fishing comes into its glory in August and September when flows drop to 1,000 cfs or lower and provide good, safe wading. Flows between 300 and 600 cfs are ideal.

The Best Hatches

The South Fork's best hatches begin on opening day the Saturday of Memorial Day weekend. The flows are usually high, but if you fish from a hard boat, raft, or pontoon boat, you can pound the banks with caddis and stonefly drys. Golden Stones and Salmonflies may be hatching, especially during June and early July, so pack large imitations, including Orange Stimulators, Cat Pukes, Improved Sofa Pillows, Foam Stones, and orange Madam Xs. Use heavy tippets short, 6-footers tapered to 1X or 2X. You'll need the diameter to hold the large fish in heavy flows.

Fly angler fishing the South Fork of the Boise River in winter
(Greg Thomas photo)

Caddis have all-day importance during June, and the hatches continue into July. Hatch-matching offerings should include gray and tan Elk-hair Caddis, X-Caddis, and Emergent Sparkle Pupae in sizes 14 and 16, tied to 4X and 5X tippets.

Cicadas are also important on the South Fork, and some of the best fishing I've had on the river came during a cicada blitz. It didn't matter where I threw my fly, trout were eager to crush #6 and #8 Royal Stimulators. Side channels held hungry but wary fish in thin water; as challenging fishing as you can find. The cicada action usually peaks in July, so make sure you pack a few black cicada patterns. When fishing any cicada pattern, I tie a #16 or #18 Pheasant-tail Nymph or caddis pupa dropper off the hook bend. It also pays to wade slowly and carefully when fishing the shallows, or you'll put the fish down.

Other places to fish during hatches include eddies, backchannels, foam lines, and grassy banks. Being patient and observant pays dividends Before stepping into the stream, use your eyes. Keep your fly in a line guide and watch the water. Rises can be subtle Fish feeding on emergers may leave just a swirl, never actually parting the surface with a nose, tail, or dorsal fin. Polarized glasses are standard gear.

As the stonefly and caddis hatches dissipate, expect Pale Morning Duns and, more important, Pink Alberts (Epeorus albertae) on the water during the afternoon hours.

PMDs provide decent action on standard patterns such as Sparkle Duns, Compara-duns, various cripples, and Parachute Adams, but it's the Pink Albert that truly shines. The Pink Albert emergence occurs on most afternoons in July and August, just as the air temperature rises into the 80- to 100-degree F. range. If you arrive during Pink Albert time, imitate the insects' most notable behavior The bugs twist free from their nymphal shucks near the river bottom and swim to the surface as fully formed adults. Trout key on this stage. A #16 emerger pattern with exposed wings should be your go-to fly during this hatch. If trout start taking duns off the surface, try a #16 Pink Albert Thorax.

In September, as summer changes to fall, the South Fork produces massive Baetis hatches. This small bug produces superb fishing for seasoned anglers who treasure late-season dry-fly opportunities. After being fished hard all summer, the 'bows are wary, and the low flows (often down to 300 cfs or less) increase this guarded behavior.

Look for afternoon Baetis hatches in late September, October, and November. They return in February and are profuse in March, especially on nasty days with rain and sleet. This is the time to find solitude on the river, losing yourself amid the hatches and large rising fish.

I've fished the South Fork's Baetis hatch many times, and it's a great event. Armed with 5X tippet and a variety of Baetis imitations including #18 and #20 Parachute Adams, Thorax Baetis, and Baetis Compara-duns I've spent afternoons on the river when trout came nearly one after the other, with rising fish always in view, sometimes in pods where their "popping" rises were audible over the river's purr. On the best days I've taken dozens of trout ranging between 12 and 17 inches, each beautifully colored, with solid red slashes on their sides.

In addition to Baetis, the South Fork serves up excellent midge hatches, matched under the surface with #20 Brassies, Copper Johns, and Zebra Midges. When fishing midges, concentrate on relatively slow water and run tandem rigs under an indicator. Carry a few Griffith's Gnats in case the trout take to the surface to dine on adult midges.

During the winter, a #4 or #6 Egg-Sucking Leech draws attention, as do Rabbit Strip Leeches and Girdle Bugs. You may hook a (threatened) bull trout. Simply land and immediately release the fish without handling it.

Timing a trip to the South Fork is critical. You don't want to arrive in April or the first three weeks of May when the river is closed. And, if you must wade, you may want to postpone a trip until September or October. My favorite time to fish the river, when I've both floated and waded efficiently, is in mid-July when the dry-fly action is grand.

I've also spent some of the best hours of my life in November and, again, in March fishing Baetis hatches in that marvelous, undeveloped canyon, sometimes standing atop Indian Rock, peering into a river full of fat rainbows sipping delicate mayflies off the surface.

I suppose what makes the South Fork such a memorable river is that, in an age when the banks of Western streams are growing trophy homes like willows, this tailwater gem remains what it always has been a remote, undeveloped desert oasis where anglers find big, wild rainbows and places to pitch a tent the way they could in the old days.

Greg Thomas is the author of Fly Fisher's Bible Montana (Frank Amato Publications, 2006) and Best Flies for Idaho (Greycliff Publishing Company, 2000). He lives in Ennis, Montana.

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