September weather, stream flows, and summary
September fly fishing on the Madison River covers all the bases, but it is also a month of transition. Early on grasshoppers abound, by the middle of September the potential for Blue Winged Olive mayflies increases, and by month’s end prospecting with large streamers could produce a trophy-sized brown trout. All of this while the river shifts from long summer days to fall days filled with crisp mornings and warm afternoons backdropped by trees ablaze with changing leaves.
Throughout the month the daily high temperatures have a broad range—from 83 degrees F in early September to below 70 degrees F by month’s end. Measurable precipitation is similar to August coming in at around 1”. The potential for the season’s first snowfall exists in September as average monthly snowfall is 0.2”. These distinct weather changes are good news to anglers wanting to fly fish the Madison River in September.
Stream flows are consistent and clear in September and water temperatures are very fish-friendly. The Lower Madison—the river downstream of Ennis Lake and Madison Dam in the Beartrap Canyon—becomes a viable option again after the warmer weather of July and August has passed. Because the two sections, the Lower Madison and the Upper Madison, have differing characteristics their fishing will reflect these differences. The Lower Madison, because of higher water temperatures than the Upper Madison, may have a longer duration of good terrestrial fishing. While the Upper Madison, because the water temperatures will cool before than the Lower Madison, may see Blue Winged Olives (BWOs) sooner.
Because the month begins feeling like summer and ends feeling like fall, September on the Madison River is a cornucopia of all things that make the Madison River so great—consistently good large dry fly fishing; number-crunching nymphing; technical small dry fly fishing to rising trout; and streamer fishing for the big ones.
The transition from summer to fall is usually easy to predict as by the middle of the month the first major cold front of the season passes through. For a few weeks after the cold front tidbits of summer-like angling may still exist with the occasional grasshopper bite, but the middle of the month ensures fall and it's cool mornings, tree leaves full of color, and spunky trout are in full force on Montana’s Madison River.
September fishing: what to expect
Fly fishing the Madison in September is like a grand buffet of all things great about Montana fly fishing, but it is very important to differentiate between early in the month and later in September. Like April, except in reverse, the fishing changes dramatically during the month. In early September the potential for any measurable hatch is small, yet the possibility of consistently good terrestrial fishing is high. Later in the month, the terrestrial fishing opportunities dwindle but the prospect of BWOs and aggressive brown trout increases.
A typical day fly fishing the Madison River in early September can feel like a summer day. Fish can be very active before the sun is high in the sky. Terrestrials may be prevalent already and starting with a dry fly is a good idea. However, because the nights are cool and it may take awhile for the morning chill to subside, a tandem nymph rig, a slowly drug or swung streamer may entice more fish than a dry fly. As the air temperature warms and the sun also warms the riverside grasses and trees, terrestrials become more active. This is the challenge of early September: because there is no obvious hatch to signal that trout are willing to eat dry flies, a leap of faith must occur. For anglers willing to commit to fishing terrestrials, early September can be the best few weeks of the year.
A typical day fishing the Madison in late September is quite contrary to early September. A later start is preferred as nights in late September are longer and cooler than early in the month. Hatches of BWOs can occur, but usually do not begin until mid- or late-morning. Anglers seeking large brown trout may consider an early morning start, dragging or slowly stripping large streamers, but with the cool water temperatures of late-September the early bird rarely gets the worm.
By mid-September and the onset of fall, brown trout begin to prepare for their spawn and become more aggressive, actively seeking out prey while also protecting their territory. Anglers committed to fishing streamers can find some of the largest brown trout of the year. The Lower Madison River outside of Bozeman is notorious for producing some big browns in late September and any streamer-loving angler should spend some time on this unique section of river.
Where to find September trout on the Madison
Because fishing in September on the Madison River is like fishing two seasons—summer and fall—the river’s trout are found in a variety of places. Early September fishes more like summer and the second half of the month will fish more like fall, plus any type of hatch in early September is a rarity.
Nymphs—mayfly, caddis, and stoneflies—are active in the Madison River year-round. In early September’s summer-like conditions, trout are most likely to be found in subsurface holding lies. If fishing tandem nymph rigs in early September, focus on the deeper water near shallow water, behind or in front of structure, or any place that can provide cover from predators or fast currents.
If fishing terrestrials, most of them are size 4 to 14, so a trout may swim out of a subsurface lie to eat a grasshopper, ant, or beetle. Trout will expend vital energy and take the risk from deep cover because a morsel as large as a grasshopper, ant, or beetle may not pass by again that day.
As the middle of the month comes and October gets closer, hatches of BWO mayflies commence and brown trout can begin to grow aggressive prior to fall spawning. BWO nymphs can be found throughout the river. When a strong hatch occurs, look for trout in slower currents and “softer water” such as the inside of river bends, seams behind rocks, and slower runs below riffles.
Brown trout can be found in the usual predator hangouts—deeper water near shallow water, hiding near structure, or along a cut bank. Although most brown trout will spawn in October or November, a few browns may begin spawning in late September. They may be found on their redds on shallow gravel bars. Please avoid targeting spawning trout when they are encountered.
Important September hatches
September is unique because it is one of the best months to fly fish The Madison, however for the first half of the month few hatches occur. But for the second half of the month, there are generations of anglers who eagerly anticipate the Madison River’s prolific hatches of Blue Winged Olives (BWOs).
The Madison River is home to a healthy population of stoneflies, mayflies, and caddis whose nymphs are active year-round, so even if the insects are not hatching, the nymphs are available to trout.
The first half of September sees reliable terrestrial fishing. Terrestrials are insects that live the entirety of their life on land. Grasshoppers, ants, spruce moths, beetles, spiders, crickets, and any other land-dwelling insect that may inadvertently find its way into the river could end up as trout food.
After the first cold front passes through, typically around September 15th, fall BWOs hatch in abundance. These mayflies are slightly smaller than their spring season cousins. Ranging in size from 16 to 22, these insects will emerge by late-morning or early afternoon and provide a few to several hours of dry fly fishing opportunities.
Rounding out the hatches for September include a few caddis hatches in early September and the occasional early October caddis in late-September.
Madison River fly box for September
BWO dry flies sizes 16 to 22
BWO emergers sizes 16 to 20
BWO nymphs sizes 16 to 20
Grasshoppers sizes 4 to 14
Ants in brown, cinnamon, and black sizes 12 to 20
Beetles in black sizes 10 to 18
Stonefly nymphs in brown and black sizes 4 to 10
Caddis pupae sizes 14 to 16
Caddis CDC emergers sizes 14 to 16
Caddis dry flies with dark grey, black or brown bodies sizes 14 and 16;
October caddis size 8
Crayfish patterns sizes 2 to 8
Sculpin patterns sizes 2 to 6
Streamers in olive, black or brown sizes 2 to 6