The calendar is the first regulation
Idaho Fish and Game's 2025 to 2027 rules treat the South Fork below Anderson Ranch Dam as its own water, from Neal Bridge on Forest Road 189 upstream to the dam, and they split the year into three parts. From December 1 through March 31, trout are catch-and-release. From April 1 through the Friday before Memorial Day weekend the river is closed to all fishing. From the Saturday of Memorial Day weekend through November 30, you may keep two trout, and none may be under 20 inches. All year, bait is not allowed and every fly or lure may carry only one barbless hook.
The spring closure is the one that catches visitors out. Idaho Angler, the Boise shop, marked the South Fork closed in every report from April into May 2026 and flagged the opener, Saturday, May 23, as one of the fishing events of the season. If you hook a bull trout anywhere in the system, it goes back: the regional general rules make bull trout catch-and-release.
What Anderson Ranch sends down
Everything on this river depends on what comes out of the dam, and the gauge on this page, USGS 13190500, sits right there. Its long record, from 1943, shows a tailwater pattern with an irrigation summer: median monthly flow of about 305 to 325 cfs from October through March, rising to about 770 in April, 1,760 in May, 2,065 in June and 1,555 in July, then falling to about 1,120 in August and 480 in September. The lower gauge at Neal Bridge, near the bottom of the special-regulation reach, stopped reporting in 2019, so the dam gauge is the working number.
When the summer releases are up, this is a float river. Idaho Angler's July 2026 report said wading was still tough at the water levels then running and expected that to change toward the end of summer and into fall.
Three hundred cfs and the fall wading window
The band on this page comes straight from the shop's fall reports. In mid-September 2025 Idaho Angler wrote that flows had dropped below 300 cfs and opened up wading; in early October it called flows around 300 cfs safe and productive; and in mid-October it described 300 cfs as optimal for wading. That lines up with the gauge's own record, which settles near 300 to 320 cfs from October through winter. The 250 to 400 cfs range here brackets that.
On September 24, 2026 the dam gauge read about 304 cfs, right in that window and below its September median. Go in the fall with a 4X tippet for most nymphs, which is what the shop recommends, and expect the fish to be spread out in water you can reach on foot.
From salmonflies to October caddis
The river opens cold. Idaho Angler's May 2026 preview said to plan on nymphing early, with fish stacked and eating subsurface, and listed blue-winged olives, baetis, some caddis and a mix of PMDs as the early menu, with salmonflies close behind. By July the dry-fly game was on: big foam on top, Chubby Chernobyls, Water Walkers, Dancin Rickys and cicada patterns, with a few salmonflies still around. The shop warns that when fish gorge on a heavy hatch they can go off the bite for 12 to 24 hours, which is why some days are excellent and others empty.
Fall brings the October caddis. The September and October 2025 reports put orange and brown Elk Hair Caddis and Burkus October Caddis at the top of the list, with smaller dark caddis, BWOs and midges filling the gaps. PMDs lingered into mid-October on the right days, and the shop mentions pink mayfly patterns such as the Pink Hackle Stacker working then. When nothing is hatching, a stonefly nymph as the heavy fly with a caddis pupa or Pheasant Tail behind it is its standard rig.
Winter in the canyon
The catch-and-release months are not dead time. Idaho Angler's December 2025 report had flows fluctuating around 300 cfs, the canyon cooling fast, and excellent blue-winged olive emerger activity on overcast days, with afternoons fishing best as the water warmed. By late February 2026 the river sat just above 300 cfs, midges, baetis and olives were giving the most consistent dry-fly fishing, and Skwala stoneflies were starting to show. Nymphing a heavy stonefly with a small Pheasant Tail, Frenchie, Rainbow Warrior or Blowtorch behind it was the dependable method, with small sculpin streamers also taking fish.
The shop's other winter note is practical: be careful getting into the canyon once snow builds up on the access.
Above the reservoir is different water
Do not carry the tailwater rules upstream. The South Fork above Anderson Ranch Reservoir falls under Fish and Game's Magic Valley Region instead of the Southwest Region, and its special section, from the mouth of Beaver Creek up to the mouth of Big Smoky Creek, has a two-trout limit with none under 14 inches, no bait and one barbless hook. The rest of the upper river runs on Magic Valley general rules. USGS keeps a gauge near Featherville on that upper river, which read about 189 cfs in late September 2026. Anderson Ranch Reservoir itself has a 15-kokanee daily limit.