September Fishing on the Stillwater River

September weather, stream flows, and summary

The Stillwater River valley in September is unique place with a special river flowing through it—with a river bottom ablaze with the colors of changing cottonwoods and the peaks of the Beartooth Mountains continually dusted with fresh snow, fly fishing the Stillwater River during September is as much about scenery as it is about plenty of wild trout eager to eat well-presented flies. 

Stream flows run consistent and clear in September and water temperatures are very fish-friendly. September’s stream flows average around 500 cubic feet per second (cfs) and in most years stay around 500 cfs throughout the month. Occasionally they dip to below 300, but fishing is still viable at flows of 300 cfs. Floating anglers will struggle at levels below 500 cfs, especially on the river upstream of Absarokee. Downstream of Absarokee floating is possible but be prepared to drag the raft across many shallow areas. 

September on the Stillwater River is ideally suited for walking-and-wading anglers as many of the fish are concentrated in runs, pools, or deep water downstream of riffle corners. However, because the streambed is made up of boulders and large rocks, walking-and-wading requires a good level of fitness and patience. 

September weather on the Stillwater River covers the four seasons—spring, summer, fall, and even winter. 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 higher than in August, piling up to about 1.4”. The potential for the season’s first snowfall exists, especially on the river near its headwaters in the Beartooth Plateau. In the lower sections near Absarokee the average monthly snowfall is 0.3” but further south—as the river gets closer to the Beartooth Plateau and Yellowstone National Park—that amount climbs to almost 1.2”. These variances in weather serve up a wide range of fishing opportunities from the river deep in Custer-Gallatin National Forest to its confluence with the Yellowstone River near Columbus. 

Early September weather feels like summer and ends feeling like fall. But the month as a whole has consistent large dry fly fishing, reliable nymphing, hatches of Blue Winged Olive mayflies, and the possibility of a trophy brown trout on a streamer. 

September fishing: what to expect

The Stillwater River in September is a fishery with two distinct personalities. As the weather changes in the middle of the month from summer-like conditions to more fall-like conditions, it is important to adjust how fishing tactics. 

Fly fishing the Stillwater River in early September feels a lot like fly fishing the river in late August—cool mornings and hot days. Fish may be active eating tricos or grasshoppers well before the sun is high overhead. Starting the day with 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. Choose beadheads in size 16 and 18, imitating small mayfly nymphs or caddis pupae. For streamers, size 6 white or black articulated streamers are best. 

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. 

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

Beginning the last week of September, anglers seeking a few trophy-sized brown trout may consider the lower sections of the Stillwater River. Large brown trout may begin to migrate into the Stillwater River from the Yellowstone River to begin the process of spawning. Size 2 to 6 black, brown, olive streamers may entice a few of these large trout. 

Where to find September trout on the Stillwater River

Finding trout on the Stillwater River in September is about finding the food. But, with a smaller amount of hatches than earlier in the season, anglers on the Stillwater River should prospect with attractor dry flies or tandem nymph rigs more so than expect to see rising trout. 

When fly fishing the Stillwater River in September, aim to get flies 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 a deep hole.

The exception to focusing on structure rather than looking for specific fish occurs when a strong hatch of Blue Winged Olive (BWOs) mayflies happens. BWO nymphs can be found throughout the river, but 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. 

Because the Stillwater River has an abundance of pocket water and fast runs, 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 Stillwater River’s faster pockets but the river’s trout are likely to only feed on the dry flies in 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. 

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 Stillwater River are minimal compared to the summer months. Because early September feels more like summer, the first two weeks of September experience very little hatches. As temperatures drop and fall season approaches, the second half of the month experiences solid hatches of Blue Winged Olive (BWOs) mayflies. 

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 finds its way into the river could end up as trout food.

Trico mayflies may still hatch in early September and skilled anglers can find trout sipping tricos in the early morning hours. Trout may feed on trico nymphs, but it is more common for Stillwater River trout to feed on larger BWO or caddis nymphs or pupae. 

It isn’t until the first cold front passes through that the flip is switched and fall BWOs can hatch. This typically occurs 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. 

Stillwater 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 white, black, or brown sizes 2 to 6