September Fishing on the Yellowstone River

September weather, stream flows, and summary

September on the Yellowstone River runs the gamut of fly-fishing opportunities. From fishing terrestrials early in the month to hatches of Blue Winged Olives and streamer fishing for aggressive brown trout later in the month, September fly fishing on Montana’s Yellowstone River is uniquely diverse. September is a month of transition on the Yellowstone. As summer fades into fall, the mornings are crisp followed by warm afternoons, and the riverside trees come alive with color as leaves change.

Weather, like the fishing, is varied. Throughout the month the daily high temperatures have a broad range—from 83 degrees F in early September to around 65 degrees F by month’s end. This temperature swing is the largest of the angling season. Measurable precipitation is a little higher than August, piling up to about 1.5”.  The potential for the season’s first snowfall exists in September as average monthly snowfall is 0.3”. These variances in weather allow for a wide range of fishing on the Yellowstone River.

Stream flows run consistent and clear in September and water temperatures are very fish-friendly. The Yellowstone River has one major tributary, the Lamar River in Yellowstone National Park, that can cause muddy water on the Yellowstone River. If a thunderstorm drops substantial rain in a section of the Lamar River drainage, rain causes muddy water in the Lamar and that muddy water eventually makes its way to the Yellowstone River.

Early September weather feels like summer and ends feeling like fall. During this month the Yellowstone 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 the possibility of a trophy brown trout on a streamer.

September is a transition time for the Yellowstone River. The change from summer to fall usually occurs in the middle of the month after the first major cold front of the fall 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 with its cool mornings and trees full of color arrives and is here to stay.

September fishing: what to expect

Similar to April, the fishing on the Yellowstone River changes dramatically throughout 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. Fly fishing the Yellowstone River in September is the perfect sampler of all things exciting about Montana fly fishing. However, it is very important to differentiate between early in the month and later in the month.

Fishing the Yellowstone 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 abound and starting the day fishing a dry fly is a good idea. The nights are cool and it may take a few hours for the morning chill to subside—September mornings are ideal sweatshirt weather. Fishing a tandem nymph rig or slowly dragging or swinging a streamer may entice more fish to strike than rise to a dry fly.

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.

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 Yellowstone River downstream of Livingston and east to Columbus is notorious for producing some big browns in late September.

A typical day fishing the Yellowstone 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 BWOs 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, then dragging or slowly stripping large streamers. With the cool water temperatures of late-September the early bird rarely gets the warm.

Where to find September trout on the Yellowstone

September on the Yellowstone River can feel like two seasons. Because of this, the 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—the river’s aquatic insects are just not primed for hatching—but the surrounding grasses are still full of grasshoppers, ants, and beetles.

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.

Nymphs—mayfly, caddis, and stonefly nymphs—are active in the Yellowstone 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.

As the middle of the month comes and October gets closer, hatches of BWO mayflies commence. Brown trout can also 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 not the end of summer on the Yellowstone River, especially in the first half of the month. Because the weather can still be warm, 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 their way into the river could end up as trout food.

A variety of hatches occur on the Yellowstone 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 mayflies are abundant.

The Yellowstone River boasts a healthy population of stoneflies, mayflies, and caddis. These nymphs are active year-round, so even if the insects are not hatching, the nymphs are available to trout.

Fall BWOs can hatch after the first cold front passes through, typically around September 15th. 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.

Yellowstone 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

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