September Fishing on the Big Hole River

September weather, stream flows, and summary

September on the Big Hole River comes as a pleasant welcome to many anglers. As the heat of summer wanes early in the month, hatches begin to return as water temperatures cool. But that doesn’t mean summer has fully vacated the Big Hole River in September. The month features a little bit of summer, a little bit of fall, and in some years even a little bit of winter. The fishing follows suit with opportunities to fish terrestrials, head hunt during hatches of Blue Winged Olive (BWOs) mayflies, or chase aggressive brown trout with streamers. 

September weather on the Big Hole River can dish-out summer one day, fall the next day, and winter the last day. 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 1.3”.  The potential for the season’s first snowfall exists, especially on the river upstream of Dewey. Near Twin Bridges the average monthly snowfall is 0.1” but further upstream—as the river gets closer to its headwaters near Wisdom—that amount climbs to almost 1”. Because the weather can be so varied in September, the fishing can as well and anglers need to be prepared for a diverse fishery with changing weather patterns every few days.  

The first half of the month has average stream flows around 300 cubic feet per second (cfs) but by month’s end stream flows actually rise above 380 cfs because less water is diverted from the river to irrigate rancher’s hay fields for feed for livestock. In some years, especially in the lower sections of the river, “hoot owl” restrictions are in place. This means fishing must stop after 2 pm as water temperatures become harmful to any caught fish. Fortunately, in most years by the second week of September stream flows have increased and the cooler weather of September causes water temperatures to drop below 66 degrees. 

This transition typically occurs after the first major cold front passes through, making September a month of change—but on the positive side of things—on the Big Hole River. For a few weeks after the first 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, trees full of color, and consistent hatches of Blue Winged Olive mayflies is here to stay. 

September fishing: what to expect

During September fly fishing on the Big Hole River changes dramatically, with a distinct difference between early in the month compared to later in the month. From the headwaters section above the North Fork to the river’s confluence with the Beaverhead River near Twin Bridges, all 150 miles of the Big Hole River should experience exceptional windows of fishing activity. 

Although the Big Hole River flows for 150 miles and contains a few distinct sections, during September the differences between the sections are less pronounced compared to other months. Rather than differentiate between the various sections, it is more important to acknowledge the changes within the month of September. 

Throughout the various sections, fly fishing the Big Hole River in early September behaves more like summer than fall. Fish may be active before the sun is high overhead. Start the day fishing a dry fly 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.

For nymphs choose mayfly or caddis patterns in sizes 12 to 18, preferably bead heads. For streamers, choose beadhead or conehead in olive or yellow and brown patterns in sizes 2 to 6. Be sure to have a few articulated patterns, such as a Circus Peanut or Sculpzilla. 

In early September, throughout the day, terrestrial fishing should improve on the Big Hole River 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. 

Typically between September 15th and 20th, the first major cold front of the fall season passes through. When this occurs, brown trout begin to prepare for their spawn and become more aggressive. These larger trout actively seek out prey. Anglers committed to fishing streamers can find some of the largest brown trout of the year. This is especially true on the river around Melrose. 

In late September when fishing the Big Hole River, a later start is preferred. After September 15th nights are markedly 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 Big Hole River

The Big Hole River in September has many personalities. As daily high air temperatures are lower than August, water temperatures in September are also lower. However, because early September can still experience warm daily high temperatures, trout can be found in a variety of places. Like other months, trout will be found near the bulk of available food and can migrate throughout various river habitats on a daily basis. 

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 because the month’s aquatic insects tend to desire colder water temperatures. But, because the river’s surrounding hay fields are still thick with grass, grasshoppers, ants, crickets, and beetles all provide food for trout. A few sporadic spruce moths may hatch, but most dry fly fishing in early September will be with grasshoppers, ants, crickets, and beetles.

Nymphs—mayfly, caddis, and stonefly nymphs—are active in the Big Hole 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. 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 first 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 Big Hole River in September. Terrestrials and spruce moths dominate early in the month and Blue Winged Olive (BWOs) mayflies can be abundant in the second half of the month. Although not related to any hatching insect, brown trout begin to get aggressive in September as they prepare to spawn in October. 

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.

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. 

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

Big Hole 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