September Fishing on the Gallatin River

September weather, stream flows, and summary

With its banks lined with the fall colors of changing cottonwood leaves and the peaks of the Gallatin and Madison Ranges dusted in fresh snow, the Gallatin River in September is quintessential Montana fly fishing. The potential for dry fly fishing exists every day of the month—grasshoppers and spruce moths early and Blue Winged Olives late—fly fishing the Gallatin River in September is full of ingredients that provide some summer-like fishing and some fall-like fishing. 

September weather on the Gallatin River can dish out heat, cold, rain, and snow. Throughout the month the daily high temperatures have a broad range—from 80 degrees F in early September to around 60 degrees F by month’s end. Precipitation is a little higher than August, piling up to about 1.8”.  The potential for the season’s first snowfall exists, especially on the river near Big Sky and in Yellowstone National Park. Near Bozeman the average monthly snowfall is 0.3” but further south—as the river gets closer to Yellowstone National Park—that amount climbs to almost 1”. These variances in weather serve up a wide range of fishing opportunities. 

Streamflow's run consistent and clear in September and water temperatures are very fish friendly. The Gallatin River has a major tributary, the Taylor Fork, that can cause muddy water on the Gallatin River. If a thunderstorm drops substantial rain in a section of the Taylor Fork drainage, rain causes muddy water in the Taylor Fork and that muddy water eventually makes its way to the Gallatin River.

Early in September the weather feels like summer and late in September the weather feels like fall. In September the Gallatin River features many of the things that make fly fishing enjoyable—reliable large dry fly fishing; consistent nymphing; hatches of Blue Winged Olives can make for technical small dry fly fishing; and on the lower river near in Gallatin Valley, the potential for a trophy-sized brown trout. 

The change from summer to fall usually occurs in the middle of the month after the first major cold front passes through, making September a month of transition on the Gallatin River. 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 with its cool mornings and trees full of color arrives and is here to stay. 

September fishing: what to expect

The fishing on the Gallatin River changes dramatically throughout the month. With the exception of a few sporadic hatches of spruce moths early in the month the potential for any measurable hatch is small, but consistently good terrestrial fishing still exists. Later in September, the terrestrial fishing opportunities dwindle but the prospect of BWOs and aggressive brown trout increases. Because fly fishing the Gallatin River in September can be quite varied, it is important to differentiate between early in the month and later in the month. 

Fly fishing the Gallatin River in early September can feel like a summer day. Fish may be active before the sun is high overhead penetrating into the water. Starting the day fishing a dry fly is a good idea because grasshoppers, ants, beetles, and spruce moths may still be prevalent. Nights in September are cool, taking a few hours for the morning chill to subside. A tandem nymph rig or slowly dragged or swung streamer may entice more fish to strike than might be willing to rise to a dry fly. 

Throughout the day, terrestrial fishing should improve as the air temperature warms and the sun warms the riverside grasses and trees. For anglers willing to commit to fishing terrestrials, early September can be the best few weeks of the year. But that can change in a matter of days if a cold front passes through. 

After the first major cold front passes through, usually by mid-September, and brings on the start of fall, brown trout begin to prepare for their spawn and become more aggressive. These larger trout actively seek out prey while also protecting their territory. Anglers committed to fishing streamers can find some of the largest brown trout of the year, this is especially true once the river leaves Gallatin Canyon and enters Gallatin Valley. 

A typical day fishing the Gallatin River in late September is quite contrary to early September. A later start is preferred as nights in late September are longer and cooler than earlier in the month. Hatches of Blue Winged Olive (BWOs) mayflies can occur, but usually do not begin until mid- or late-morning. Anglers seeking large brown trout may consider an early morning start as well and drag or slowly strip large streamers. With the colder water temperatures of late-September, the early bird rarely gets the worm as most trout are active later in the day. 

Where to find September trout on the Gallatin River

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—the river’s aquatic insects are just not primed for hatching—but the surrounding grasses are still full of grasshoppers, ants, and beetles. Because the weather in September varies, the trout are found in a variety of places. A few sporadic spruce moths may hatch, but most dry fly fishing in early September will be with grasshoppers, ants, and beetles. 

Nymphs—mayfly, caddis, and stonefly nymphs—are active in the Gallatin River year-round. In early September’s summer-like conditions of bright sunshine, trout are most likely going 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.

As the middle of the month comes and October gets closer, terrestrial fishing will dwindle but hatches of Blue Winged Olive (BWO) mayflies commence. 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. With the season’s first cold front, brown trout also begin to grow aggressive prior to fall spawning. 

After the cold front passes, look for brown trout 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

A variety of hatches occur on the Gallatin River in September, making this one of the best months to fly fish the river. The first two weeks of September actually experience very little hatches, however during the second half of the month hatches of Blue Winged Olive (BWOs) mayflies are abundant. 

Fall BWOs can hatch after the first cold front passes through, typically around September 15th. A cool, slightly rainy or overcast day is ideal for a strong emergence of BWOs. 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 can provide a few hours of dry fly fishing opportunities. 

In the first half of the month, the weather can still be warm and terrestrial fishing can be quite reliable. 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.

Rounding out the hatches for September include a few caddis hatches in early September and the occasional early October caddis in late-September. 

Gallatin River fly box for September

Grasshoppers sizes 4 to 14

Crickets in sizes 8 to 12

Ants in brown, cinnamon, and black sizes 12 to 20

Beetles in black sizes 10 to 18

Spruce moths mottled in tan or grey in sizes 12 to 16

BWO dry flies sizes 16 to 22

BWO emergers sizes 16 to 20

BWO nymphs sizes 16 to 20

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