The Madison River below Ennis Lake is one of the most storied trout streams in the American West. It's also one of the most thoroughly gauged. USGS station 06041000, near McAllister, has been recording flow and temperature data continuously — and that data tells a story about when, where, and how to fish this river that goes well beyond conventional wisdom.
What follows is a guide to the Madison built on real numbers. Every flow figure, temperature range, and seasonal pattern referenced here comes from actual USGS gauge readings. The hatch data is sourced from Riffle's database of 33 documented hatches for this river. This is the Madison as the data sees it.
The River at a Glance
The Madison below Ennis Lake is a tailwater, which fundamentally shapes its character. Water released from the bottom of the reservoir is cold, clear, and relatively stable compared to freestone rivers. This gives the Madison two properties that make it exceptional: a long fishable season and a moderated runoff that rarely blows it out completely.
| Metric | Value | What It Means |
|---|---|---|
| Winter baseflow | 1,150–1,300 CFS | Remarkably stable, fishable year-round |
| Peak runoff | ~2,870 CFS | Mid-May 2025 — high but manageable |
| Summer low | ~1,060 CFS | Mid-August, annual minimum |
| Coldest water | 31–36°F | January — near-freezing, midges only |
| Peak water temp | 70.9°F | Mid-July — trout stress zone |
| Documented hatches | 33 species | 12 classified as major |
Reading the Flow Calendar
The Madison's annual flow cycle is more compressed than most western rivers. Because Hebgen and Quake Lakes absorb a significant portion of the snowmelt, runoff on the lower Madison doesn't produce the violent spikes you see on freestone rivers like the Yellowstone or Gallatin.
In 2025, flows held steady around 1,200–1,400 CFS through March and most of April. The first real rise started in late April, climbing through 1,700 CFS. Peak hit around 2,870 CFS in mid-May, with a secondary pulse pushing 2,800 in early June. By late June, flows had dropped back below 1,400 and continued falling to the annual low of 1,060 CFS in mid-August.
That means the runoff window on the Madison is roughly May 10 through June 10 — about four weeks. And because this is a tailwater, the river often remains fishable even during that window, especially in the upper sections closer to the dam. Compare that to the Yellowstone, where runoff can render the river unfishable for six to eight weeks.
After runoff, flows stabilize through the summer and fall at 1,100–1,200 CFS. Winter dam releases keep flows remarkably consistent through December, January, and February. The Madison is one of the few rivers in Montana where you can count on roughly the same flow in January as in September.
The Temperature Story
If flow gives you the structure of the season, temperature gives you the biology. The Madison's temperature cycle defines when hatches fire, when fish feed, and when you should stay home.
| Month | Avg Temp | Range | What's Happening |
|---|---|---|---|
| Jan–Feb | 34°F | 31–39°F | Near-freezing. Midges on warm afternoons. |
| Mar | 38°F | 34–41°F | Still cold. Little Black Stoneflies emerging. |
| Apr | 47°F | 41–52°F | Skwalas and BWOs. Nymphing turns on. |
| May | 55°F | 51–65°F | Prime window opens. Mother's Day Caddis, early PMDs. |
| Jun | 64°F | 59–69°F | Peak. Salmonflies, PMDs, Golden Stones, Green Drakes. |
| Jul | 68°F | 66–71°F | Warm. Stress zone days. Morning/evening only. |
| Aug | 68°F | 66–70°F | Sustained warmth. Hoppers, Tricos at dawn. |
| Sep | 63°F | 60–67°F | Prime returns. BWOs, Mahogany Duns, October Caddis. |
| Oct | 49°F | 41–58°F | Cooling. Brown trout pre-spawn. Streamer season. |
| Nov–Dec | 38°F | 32–44°F | Cold. Short midge windows, then ice-up. |
The critical insight: the Madison has two prime windows, not one. The first runs from mid-April through late June, as water warms through the 47–65°F sweet spot. The second arrives in September, when temps drop back from the summer stress zone into the low 60s. July and August are the gap — fishable, but only at the margins of the day.
The Hatch Calendar
The Madison supports 33 documented hatch species, 12 of them classified as major. That density is exceptional — it means that from March through November, something is almost always hatching. Here's how the major hatches map to the temperature and flow data.
Early Season: March–April
38–52°F 1,200–1,400 CFS
Little Black Stoneflies are the first movers, emerging when water is still in the high 30s. By mid-April, as temps push past 45°F, Skwala Stoneflies appear — the first big dry fly opportunity of the year. A Chubby Chernobyl in olive, size 8–10, is the go-to. Blue-Winged Olives start showing on overcast afternoons in late April. This is nymph-heavy fishing with occasional dry fly rewards on the right days.
The Main Event: May–June
55–65°F 1,400–2,800 CFS
This is why people fly across the country to fish the Madison. The Mother's Day Caddis kicks off in early May — dense clouds of olive caddis blanketing the river, fish rising recklessly. Elk Hair Caddis, size 14–16. Then comes the Salmonfly hatch, typically late May into early June when water hits 55–58°F. Three-inch stoneflies. Fish looking up. Chubby Chernobyl or Norm Wood Special in orange/black, sizes 4–8. Overlapping the salmonflies: Golden Stoneflies, Pale Morning Duns, and Green Drakes. June on the Madison is sensory overload — multiple hatches happening simultaneously, fish feeding on the surface, and the water at perfect temperature.
The Summer Gap: July–August
66–71°F 1,060–1,400 CFS
Water temps regularly push into the trout stress zone. The data shows multiple days above 68°F in both months, with the 2025 peak hitting 70.9°F in mid-July. Fish early — Tricos provide exceptional technical dry fly fishing at dawn, with spinner falls that demand size 20–24 patterns. By mid-morning, switch to hoppers. The Madison Valley funnels wind across open grassland, blowing terrestrials into the river all summer. A Morrish Hopper or Dave's Hopper, size 6–10, banked against the far shore is deadly. Spotted Sedge caddis provide explosive evening rises. But respect the temps — if water is above 68°F, consider the upper river near Hebgen where releases are cooler, or wait until September.
The Second Prime: September–October
49–63°F 1,150–1,200 CFS
September is arguably the best month to fish the Madison. Temps drop back into the low 60s — prime feeding range — and the crowds thin dramatically. Blue-Winged Olives return for their fall emergence, often heavier than spring. Mahogany Duns join them on cooler afternoons. The big event is the October Caddis, a size 6–10 orange monster that brings the biggest fish to the surface. By October, brown trout are staging for the spawn and become aggressive — streamers fished tight to banks and through deep runs produce the largest fish of the year. Low, clear water at 1,150 CFS means technical presentations but visible fish.
The One Pattern You Need
If you had to fish the Madison with a single fly all year, it would be a Chubby Chernobyl. It appears in the Riffle database as a recommended pattern for four different hatches on this river: Skwalas in spring, salmonflies and Golden Stoneflies in summer, and hoppers in late summer. In tan, olive, or orange depending on the season, sizes 6–12. It floats high, it's visible in fast water, and it catches fish from April through September. Drop a Pat's Rubber Legs or Zebra Midge off the bend as a dropper and you've covered 80% of the situations this river throws at you.
What the Data Actually Tells You
The conventional take on the Madison is that it's a June river — show up for the salmonfly hatch, fight the crowds, and go home. The data tells a different story.
September is underrated. Temps in the low 60s, stable flows at 1,150 CFS, three major hatches overlapping, and a fraction of the pressure. If you can only make one trip a year, the data says September.
July and August are overrated. Water temps push into stress territory regularly. The fishing can still be excellent at dawn and dusk, but midday is often too warm to fish ethically. The data shows 68°F+ readings on most days in both months.
April is underappreciated. As water warms through the 40s, Skwalas and BWOs provide quality dry fly fishing at low, clear flows with almost no one else on the river. It's cold. Bring layers. But the fish are hungry after winter.
Runoff is shorter than you think. Four weeks, mid-May to mid-June. And because this is a tailwater, it's often fishable throughout. Don't write off the entire month of May.
The Madison doesn't have secrets. It has data. The anglers who fish it best are the ones who read the gauge before they read the hatch chart.
Check current Madison River conditions — flow, temperature, and fishability score — right now with Riffle's conditions tool. Or open the app for hatch predictions, flow trends, window alerts, and the full 33-species hatch calendar.