September Fishing on the Boulder River

September weather, stream flows, and summary

As summer fades into fall, the Boulder River in September mirrors the transition of the seasons. Early September on the Boulder River feels like summer and late September on the Boulder River is definitely fall. In September the Boulder River features many of the things that make fly fishing enjoyable—reliable large dry fly fishing; consistent nymphing; hatches of Blue Winged Olives for technical small dry fly fishing; and downstream of the confluence with the West Boulder River, the potential for a trophy-sized brown trout.

Despite the numerous positives of fly fishing the Boulder River in September, anglers must commit to walk-and-wade fishing almost entirely. Because stream flows in September rarely are above 200 cubic feet per second (cfs) only very experienced kayakers float the Boulder River. Like the rest of the angling calendar, it is important to differentiate between the upper and lower sections of the river.

The upper section is the water above Natural Bridge and the lower section is the water below Natural Bridge. Above Natural Bridge the fast pocket water sections are exclusive to wading anglers and water temperatures often cool sooner than below Natural Bridge. Below Natural Bridge, the only opportunity to float fish the Boulder River during September may occur in this section. Both sections in September can see hatches of Blue Winged Olive (BWOs) mayflies, caddis, and terrestrials. 

September weather on the Boulder River can cover all seasons—spring, summer, fall, and even winter. 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 1.5”. The potential for the season’s first snowfall exists, especially on the river south near the border of Yellowstone National Park. Near Big Timber 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. 

September fishing: what to expect

September on the Boulder River is almost entirely a walk-and-wade fishery. Because of low stream flows, only very experienced kayakers can navigate the river’s myriad of shallow boulder-filled pocket waters and whitewater sections. This is often the case in both the upper and lower sections. 

Because many of the ranchers cease irrigation of their hayfields in the second half of the month—they use water from the river to irrigate the fields—stream flows in late September can be slightly higher than early September. If a large amount of rainfall occurs in late September, a very slight possibility exists for a very experienced oarsman to navigate the lower river in a specially designed raft. 

Anglers on the Boulder River in September may choose to target the upper or low sections of the river. The upper section will provide more opportunity because more water is accessible for walking-and-wading anglers. The lower section may provide more opportunities for fishing terrestrials, but because floating is non-existent, anglers must commit to walking-and-wading a great deal to cover enough water. 

Fly fishing the Boulder River in early September can feel like a summer day. Fish may be active before the sun is high overhead. 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 through a deep pool or long run 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

A typical day fishing the Boulder 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 Boulder River

Finding trout on the Boulder River in September, like August, is straight forward. Look for trout to be in front of and behind boulders or any other structure; along bank-side seams and soft currents near faster currents; drop-offs below shelfs; and around submerged structure in the many pools below a run of whitewater or large run of rapids.

The exception occurs when a strong hatch of Blue Winged Olive (BWOs) mayflies happens. 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. 

Granted, on the Boulder River there is very little ”soft water” so if a hatch of BWOs occurs, target the slicks behind rocks, the slower water at the end or edge of a deep pool, and the tailouts of a run. BWOs will hatch throughout the Boulder River’s faster pockets and whitewater but the river’s trout are likely to only feed on the dry flies in the few tidbits of slower water. 

By mid-September the prospect for the region’s first major cold front increases. When the first cold front passes, brown trout 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. Because the Boulder River flows into the Yellowstone River near Big Timber, the lower section of the Boulder River has the potential to produce a trophy-sized brown trout that has migrated upstream from the Yellowstone River. 

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

Hatches in September on the Boulder River vary greatly from early September to later in the month. Because early September is more like summer and late September is most certainly fall, a variety of hatches occur. The first two weeks of September experience very little hatches, however during the second half of the month hatches of Blue Winged Olive (BWOs) mayflies are abundant. 

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. 

Boulder 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