Montana’s Classic Stonefly Bench: The Green River and the South Fork Boise — Tailwater Stoneflies of the Intermountain West
Series: Montana’s Classic Stonefly Bench — Article 5
Not every great stonefly river announces itself.
The Deschutes canyon puts on a show — willows crawling with salmonflies, redband trout hammering dries against the basalt walls, boat traffic on the road section that reminds you why you floated past Trout Creek. The Madison in late May is an event, orchestrated and celebrated and attended accordingly. These rivers know what they are and so does everyone else.
The Green River below Flaming Gorge Dam and the South Fork of the Boise below Anderson Ranch Dam are different animals. They are remote, cold, technical tailwaters that sit in canyon country and don’t advertise. The Green runs through red rock Utah in water so clear you can count the fish before you cast to them. The South Fork Boise drops through a steep Idaho canyon that limits road access to the point where most of the fishing happens from a drift boat or by hiking in on foot. Both rivers hold brown trout that have seen enough flies to have opinions about everything — your presentation, your tippet, your drift, the specific shade of olive on your nymph.
I know the Green from the water. Several days below the dam, bent rods and wind and rain and the kind of nymphing that requires full attention because the fish that eat your Kaufmann’s Rubberlegs in that current are not small and they do not give you time to think about what comes next. Brown trout in the Green River do not negotiate. They take the fly and they go and the reel tells you everything you need to know about whether your knots were tied correctly that morning. The South Fork Boise I know from wading trips — a few hours here and there on different visits, the kind of river you keep coming back to in pieces because each piece gives you something worth returning for.
These are not easy rivers. They are rewarding ones — which in this series, as in the fishing life generally, is the better thing to be.
This is Article 5 of Montana’s Classic Stonefly Bench. The series that started on Pat Barnes’ bench in West Yellowstone has now covered Montana, Oregon, and arrives here in the canyon country of Utah and Idaho with three patterns built specifically for the demands of technical tailwater stonefly fishing.
Table of Contents
Two Tailwaters, One Shared Logic
The Green River and the South Fork Boise share a fundamental character despite being in different states on different drainages. Both are dam-controlled tailwaters with cold, stable year-round water temperatures that support large populations of educated brown and rainbow trout. Both fish best from a drift boat in high water and on foot in low, clear conditions. Both reward technical nymphing pre-hatch and precise dry fly presentation when the surface feeding starts. And both hold fish that have been fooled enough times to require something better than approximate.
The Green River:
below Flaming Gorge Dam flows through a spectacular red rock canyon in northeastern Utah — about 30 miles of fishable water divided into three sections, A through C, with Section A immediately below the dam holding the densest populations of fish and the most consistent hatch activity. The river runs cold and steady year-round, 48 to 52 degrees near the dam, which keeps the insects hatching on a near-daily cadence and the fish feeding consistently across every season. The stonefly component of the Green’s hatch calendar is not the headline act — that belongs to the PMD and BWO hatches — but the yellow sally and golden stone are present from June through midsummer, and the Kaufmann’s Rubberlegs nymph fished pre-hatch in the deep runs produces the kind of brown trout that remind you why you drove to northeastern Utah.
The river fishes best in the 800 to 2,000 cfs window. Above that range the wade options disappear and the nymph rigs need to go heavier. Before you load the truck, check the gauge — Flaming Gorge releases can change overnight and the difference between a fishable Section A and a blown-out wade is a few hundred cfs you didn’t see coming.
The South Fork Boise:
below Anderson Ranch Dam is one of the best tailwater fisheries in Idaho and one of the least written-about quality fisheries in the American West. Wild brown and rainbow trout averaging 14 to 18 inches, with fish in the 18 to 22 inch range not uncommon in the deeper canyon pools. The stonefly hatch calendar here runs from the yellow sallies in late May through the salmonfly and golden stone hatches in June and July, with October Caddis extending the surface fishing into fall. Large orange Stimulators, foam stones, and Pat’s Rubber Legs are the foundational patterns — the same logic as everything else in this series, applied to water that is more remote and less pressured than the famous Idaho rivers that get all the attention.
Tailwater Stonefly Calendar
| Hatch | River | Timing | Size |
|---|---|---|---|
| Little Yellow Stone | Green River, South Fork Boise | June–July | 12–16 |
| Golden Stonefly | Green River, South Fork Boise | June–early August | 6–10 |
| Salmonfly | South Fork Boise | Mid-June–mid-July | 4–8 |
| October Caddis | South Fork Boise | September–October | 6–8 |
Pre-hatch nymphing window:
Both rivers fish the Kaufmann’s Rubberlegs and Pat’s Rubber Legs through the cold months and the pre-hatch period — any time stonefly adults are not yet on the water. On the Green this means most of the year. On the South Fork Boise it means spring through early June before the salmonfly nymphs begin their migration to the banks. The nymph is never wrong on either river if the hatch isn’t happening. The nymph is often right when the hatch is happening too.
How to Tie the Kaufmann’s Rubberlegs Stone Nymph
Reason This Fly Exists
Randall Kaufmann built the Rubberlegs Stone Nymph to solve the pre-hatch problem on large western tailwaters — the window before adults appear when stonefly nymphs are the primary large food source for big trout and the presentation required is deep, heavy, and precise. The Green River below Flaming Gorge is exactly the kind of water this fly was designed for: cold, clear, and full of brown trout holding in deep runs where a lightweight nymph never reaches the strike zone.
The fly is not subtle. It is heavy, dark, and leggy — a convincing stonefly nymph silhouette built on a long shank hook with tungsten or lead weight that gets it to the bottom of the run where the big fish are holding and keeps it there through the drift. The rubber legs create movement that no other material replicates in slow, cold tailwater currents where the fish have time to inspect everything. On the Green River in wind and rain with high flows and brown trout that are not interested in anything small, this fly bent my rod more than once and screamed the reel every time it did.
Hook & Materials
| Component | Material |
|---|---|
| Hook | Tiemco 5263 or 200R, 3X long nymph, size 4–10 |
| Thread | UTC 140 or 70, black or dark brown |
| Weight | Lead wire or tungsten bead, appropriately sized |
| Tail | Dark brown goose biots, split |
| Abdomen | Dark brown or black chenille, medium |
| Rib | Copper wire, fine |
| Legs | Brown barred rubber legs, two per side at thorax |
| Wingcase | Dark turkey tail or black Thin Skin |
| Thorax | Dark brown or black dubbing — SLF or Haretron |
| Antennae | Dark brown goose biots, split, optional |
| Head | Thread or tungsten bead |
Weight note:
The Rubberlegs Stone Nymph needs to get to the bottom of a deep tailwater run and stay there. Use a tungsten bead at the head and additional lead wire wraps under the thorax on the Green River and other large, deep tailwaters. Underweighting this fly is the most common mistake — you will spend the drift in the wrong part of the water column and the fish will never know you were there.
Hook note:
The 3X long shank is not optional. Stonefly nymphs are long, segmented insects and the hook length is part of what makes the silhouette convincing at depth. A standard nymph hook produces a shorter, stubbier fly that doesn’t read as a stonefly nymph to fish that have been eating the real thing.
Tying Instructions
Step 1 — Weight and Thread Base
Slide a tungsten bead onto the hook and mount in the vise. Wrap fifteen to twenty turns of lead wire behind the bead and push the coils forward against it. Start dark thread behind the lead wraps and build a smooth transition — thread ramp from shank diameter up to lead diameter and back down. The thread transition keeps the lead from sliding and gives the chenille abdomen something to bind against.
Step 2 — Tail
Select two dark brown goose biots. Tie them in at the bend on opposite sides of the hook shank, tips curving outward and slightly downward — not upward. The split biot tail creates the forked tail profile of a natural stonefly nymph. Length: one quarter to one third of the hook shank. Bind each biot down individually with three tight thread wraps before adding the second.
Step 3 — Ribbing Wire
Tie in a length of fine copper wire at the tail tie-in point. Let it hang for now.
Step 4 — Abdomen
Tie in dark brown or black medium chenille at the tail tie-in point. Wrap forward in tight, touching turns to the two-thirds mark on the shank, building a fat, segmented-looking abdomen. The chenille should be full but not sloppy — each turn should be distinct. Tie off and trim.
Step 5 — Rib
Counter-wrap the copper wire forward through the chenille abdomen in five to six evenly spaced turns. The counter-wrap goes in the opposite direction from the chenille — this locks the chenille down, adds durability, and creates a subtle segmentation that the chenille alone doesn’t produce. Tie off and trim.
Step 6 — Rubber Legs
Cut two pairs of barred rubber legs — brown barring on tan or orange base is the classic Kaufmann color. Using figure-eight wraps, tie one pair of legs on each side of the hook at the front of the abdomen. Each leg should extend approximately one hook-gap length beyond the body on each side, splaying slightly downward and outward. These legs are the heartbeat of the fly in moving water — every micro-current variation moves them and the fish respond to that movement the way they respond to nothing else at depth.
Step 7 — Wingcase
Cut a strip of dark turkey tail feather or black Thin Skin, approximately the width of the hook gap. Tie it in flat on top of the hook directly behind the bead, pointing rearward over the abdomen. Do not fold it forward yet — it will be folded over the thorax in Step 9.
Step 8 — Thorax
Dub a full, slightly rough thorax of dark brown or black SLF or Haretron dubbing between the wingcase tie-in point and the bead. The thorax should be distinctly fuller than the abdomen — it represents the swollen thorax section of a mature stonefly nymph in the pre-hatch migration period. Two to three full turns of well-loaded dubbing.
Step 9 — Fold Wingcase
Pull the turkey tail or Thin Skin strip forward over the thorax and tie off directly behind the bead. The wingcase should lie flat and smooth over the top of the thorax. Trim the excess flush. A drop of UV resin or head cement over the wingcase adds durability and a slight sheen that catches light at depth.
Step 10 — Head and Finish
Build a small, clean thread head behind the bead. Whip finish twice. Hard head cement on all thread wraps.
Tying Notes
Carry in sizes 4, 6, and 8. The size 4 is for the deepest, fastest runs on the Green and South Fork Boise where the fish are holding beyond the reach of smaller flies. The size 6 covers most situations. The size 8 is for lower, clearer water and more selective fish.
Fish this nymph under an indicator set at one and a half times the water depth, with enough split shot above the fly to keep it ticking along the bottom on a natural drift. On the Green River in wind and high water, that means more shot than feels right and a longer indicator set than you’re used to. Trust the weight. The fish are on the bottom and the fly needs to meet them there.
