Troutline

South Fork Boise River

Idaho·Southwest Idaho·43.37° N, 115.53° W
Flow
1,580 CFS
SF Boise River at Anderson Ranch Dam
Water Temp
47°F
SF Boise River at Anderson Ranch Dam
Condition
Above Normal
Weather
74°F
Partly Cloudy
near Mountain Home

Insights

Flow
1,580 CFS — higher than typical
Push to the banks and softer water. Heavier flies.

The South Fork of the Boise below Anderson Ranch Dam is the tailwater every Treasure Valley angler defaults to when they want big wild trout inside a two-hour drive of Boise. Cold, nutrient-rich water pulled off the bottom of Anderson Ranch Reservoir holds the river between roughly 40 and 55°F year-round, and that stability grows a lot of fish — surveys have put densities well north of 3,000 trout per mile in the upper reach, mostly wild rainbows running 14 to 18 inches with browns mixed in and the occasional 20-inch-plus fish. It fishes twelve months a year, which is rare in this part of Idaho: when the freestones are blown out in spring or locked up in winter, the South Fork is still clear and cold.

How it fishes depends entirely on what the Bureau of Reclamation is doing with the dam, so always check the gauge before you go. Below about 600 cfs it's a wader's river — you can cross in spots, work the seams with a PMD or a hopper-dropper, and cover the first half-mile below the dam where the fish stack thickest around the Pine Tree Hole near the Granite Creek confluence. Once summer irrigation demand ramps the release toward 1,600 cfs, wading gets sketchy and the smart play is a drift boat or raft throwing big buggy dries with a stonefly nymph dropped underneath. A release change can turn a wadeable morning un-wadeable by afternoon, and it moves the fish too. The signature event is the golden stone drift in late June and July; PMDs carry the summer, and a strong midge-and-BWO shoulder-season program keeps the river fishing all winter.

The first ten miles from the dam to Danskin Bridge is roaded, with a dirt road hugging the bank and unlimited public access. Below Danskin the river drops into a roadless canyon and runs sixteen miles to Neal Bridge at the head of Arrowrock Reservoir — a Class I–II day float you don't want to start late, with very little foot access. This is a fly-fishing-only, mostly catch-and-release fishery, and no guided trips are permitted in the tailwater canyon, which keeps pressure lower than the fish counts would suggest. One thing to keep straight: "South Fork Boise" also names the freestone reach above the reservoir near Featherville — a genuinely different river (warmer, snowmelt-driven, no tailwater buffer). The fishery Boise anglers mean is the tailwater below the dam.

Species

  • Rainbow Trout
    Primary · Jun-Oct · 12-18"

    The dominant fish in the tailwater and the reason to come — wild, resident, and packed in at very high densities in the first miles below the dam. Most run 14-18 inches with fish over 20 in the mix. They see a lot of natural drifts and can get picky on the flats.

  • Brown Trout
    Common · Sep-Nov · 12-20"

    Increasingly common as you move downstream into the canyon. Fall pre-spawn is prime streamer time — the biggest browns of the year come from swinging and stripping in October and November.

  • Mountain Whitefish
    Abundant · Year-round · 10-16"

    Native and dense throughout. A reliable winter nymphing target when the trout bite slows — they hit small nymphs and scud patterns aggressively.

  • Bull Trout
    Protected · — · varies

    Native and federally protected, most likely near the Big Smoky Creek drainage above the reservoir. Zero harvest — any bull trout must be released immediately, unharmed.

Ideal wading flow300–600 CFS
Blow-out>2,000 CFS
Ideal water temp48–58°F

Fall is the top pick — browns turn aggressive, October caddis brings big fish up, flows drop back to wadeable, and the crowds thin. Summer fishes well on PMDs, caddis, and hopper-droppers but the release dictates everything, so watch the gauge. Spring brings BWOs but note the Apr 1 to Friday-before-Memorial-Day closure in the tailwater. Winter is catch-and-release midge and BWO fishing when the freestones are frozen. Because it's a bottom-release tailwater, this river is schedule-dependent, not weather-dependent — a 1,600 cfs irrigation release fishes fine from a boat but shuts down wading.

Sections

3 sections on this river

Upper Freestone — near Featherville

WadeBrown Trout · Rainbow Trout

The freestone South Fork above Anderson Ranch Reservoir near Featherville — a genuinely different river from the tailwater: warmer, snowmelt-driven, and prone to blowing out in spring runoff. Pocket water and pools with brown trout increasingly prevalent alongside rainbows, and a bull-trout zone near the Big Smoky Creek confluence (zero harvest). Fed by Forest Service roads out of Featherville.

Best for: Summer and early-fall wade fishing for rainbow and brown trout once runoff drops. Hoppers and attractor dries through the pocket water.

The Canyon — Danskin Bridge to Neal Bridge

FloatSalmon · Brown Trout · Rainbow Trout

A remote roadless canyon running about sixteen miles from Danskin Bridge to Neal Bridge at the head of Arrowrock Reservoir, with Class I–II whitewater, high walls, and big pools. Very limited foot access between the put-in and take-out — this is a day float you don't want to start late. Much less pressured than the roaded tailwater, with trophy brown trout that turn on in fall.

Best for: Drift boat or raft throwing big dries with a stonefly dropper; fall streamers for brown trout. Salmonfly and golden stone drift-boat fishing in early summer.

The Tailwater — Anderson Ranch Dam to Danskin Bridge

Wade & FloatBrown Trout · Rainbow Trout · Whitefish

Classic tailwater riffle-run-pool below Anderson Ranch Dam — cold, clear, weedy and bug-rich, with the highest trout densities on the river in the first half-mile below the dam (the Pine Tree Hole near the Granite Creek confluence is the marquee lie). A dirt road parallels the first ten miles with unlimited public access and numerous pullouts. Wild rainbow trout dominate at 14-18 inches with browns and mountain whitefish mixed in.

Best for: Wade nymphing and dry-fly for rainbow trout under ~600 cfs; drift-boat dry-dropper when summer irrigation flows climb toward 1,600 cfs. PMDs, caddis, and golden stones in summer; hopper-dropper on the banks.

Regulations

Current fishing rules and restrictions

The premier reach — Neal Bridge upstream to Anderson Ranch Dam — is effectively fly-fishing-only (no bait, one barbless hook per fly or lure) with a seasonal closure and a strict, large-fish-only harvest window. Bull trout are protected with zero harvest, and no guided trips are permitted on this reach.

  • Neal Bridge to Anderson Ranch Dam: no bait, one barbless hook per fly or lure (effectively fly-fishing-only)
  • Dec 1 – Mar 31: trout catch-and-release
  • Apr 1 – Friday before Memorial Day weekend: closed to fishing
  • Saturday of Memorial Day weekend – Nov 30: trout limit 2, none under 20 inches
  • Bull trout: zero harvest, release immediately
  • No guided trips permitted on the Neal Bridge to Anderson Ranch Dam reach
  • Upper freestone above the reservoir (Featherville and above) follows separate IDFG general/regional rules — check the Fishing Planner per section

Regulations change annually — these reflect the 2026 rules. Re-verify the exact dates and limits on the IDFG Fishing Planner before your trip, and confirm which reach you're on: the tailwater and the upper freestone are governed separately. Idaho fishing license required.

Source: Idaho Department of Fish and Game — Fishing Planner and 2026 Seasons & Rules. Regulations change annually — verify before fishing.

Access & Logistics

Getting there, fly shops, and lodging

Getting There

Mountain Home, ID

~40 min from Mountain Home, 1.5-2 hrs from Boise

Fly Shops

Camping & Lodging

Cow Creek and Tailwaters campgrounds sit on the roaded tailwater, with abundant dispersed sites along the first ten miles below the dam. No lodges directly on the tailwater — the town of Pine and Featherville (above the reservoir) offer limited services, and Mountain Home has full services off I-84.

Reach the dam via Anderson Ranch Dam Road off US-20. Walk-in wade access is right below the dam via a short trail from the dam parking lot. The roaded section runs about ten miles to Danskin Bridge; below that the canyon is float-only to Neal Bridge (Forest Road 189) at Arrowrock Reservoir. No guiding is permitted in the tailwater canyon, so Boise-area shops sell intel and gear rather than SF Boise trips. Verify winter road conditions in the Boise National Forest.

Conditions data is live from public monitoring networks. Regulations change annually — always verify current rules with your state fish & wildlife agency before fishing.

More in Idaho

View all 17 rivers

Southwest Idaho

Boise RiverID

The rare trout tailwater you can fish on a lunch break — cold, clear water off the bottom of Lucky Peak Dam runs straight through downtown Boise, paralleled the whole way by the paved Greenbelt. Wild brown and rainbow trout (browns commonly 16-20 inches), native redband, and abundant mountain whitefish. Flood-control and irrigation releases dominate: summer often pushes 1,000-7,000+ CFS and fishes poorly, but the river turns on after irrigation season ends in fall and through winter at a wadeable 150-600 CFS.

Other regions

Big Lost RiverID

A high-desert freestone under the Pioneer and White Knob ranges that turns, below Mackay Reservoir, into a short cold tailwater growing wild rainbows past 20 inches — then spreads out below Arco and sinks into the desert at the Big Lost Sinks without ever reaching the sea.

Big Wood RiverID

The freestone that runs down the spine of the Wood River Valley past Ketchum, Sun Valley, Hailey, and Bellevue — a wild, unstocked river of heavily spotted rainbows and thick lower-river browns, best fished on foot with a dry-dropper once July runoff clears.

Clearwater RiverID

A big, cold, low-elevation river that draws anglers from all over for one thing: B-run steelhead — the largest summer-run steelhead south of the Canadian border, ocean-going rainbows that average 10-13 pounds and push past 20. It's classic spey country, swung flies on broad even-depthed runs from Lewiston up through Peck and Lenore, with a shoulder-season dry-fly window for native westslope cutthroat and rainbows when the main stem drops and cools.

Fall RiverID

The Fall River is the largest tributary of the Henry's Fork, a runoff-fed freestone that tumbles off the Pitchstone Plateau in southwest Yellowstone, past a string of waterfalls, and out onto the potato ground east of Ashton. It fishes late — high and off-color into June, then a July salmonfly-to-PMD dry-fly river most Henry's Fork anglers drive right past.

Henry's ForkID

The Henry's Fork is the spring-fed tailwater that runs from Henrys Lake through Island Park and on to its confluence with the South Fork Snake near Rexburg. The slow flat water of Harriman State Park (the Railroad Ranch) is the most famous technical dry-fly stretch in the West.

Lochsa RiverID

A wild, unstocked freestone that runs right along US-12 for 70 miles of clear-water pocket water — native westslope cutthroat that eat dries two steps from the pullout, plus bull trout in the deep holes. Snowmelt-driven and treacherous to wade; it's a walk-and-wade river, not a trout float, and rarely comes into shape before mid-July.