Happy September! Here is some information from Montana Angler on what to expect this September:

September weather, streamflow’s, and summary

September fly fishing on the Missouri River is a month of transition. Early September feels more like summer yet late September—with the possibility of snow—can feel like fall or even winter. Around Labor Day grasshoppers abound, by the middle of September the potential for Blue Winged Olive mayflies increases, and by month’s end prospecting with large streamers could produce a trophy-sized brown trout. All of this as the river gravitates away from sunny summer days to fall days with cool mornings and warm afternoons backdropped by trees with changing leaves.

In September many anglers also focus attention on the water below Hauser Dam—aptly named The Land of the Giants—in hopes of landing one of the many large brown trout that migrate upstream from Holter Lake to begin to spawn. The forty-plus miles of water below Holter Dam near Wolf Creek and Craig provide floating and wading anglers a broad array of angling options.

The weather in September is diverse as well. With average daily high temperatures around 85 degrees F in early September to below 70 degrees F by month’s end, anglers need to be prepared for sunshine, rain, and even snow. Measurable precipitation accumulates at around 1.2”. The potential for the season’s first snowfall exists in September as average monthly snowfall is 0.2”. These distinct weather changes are good news to anglers fly fishing the Missouri River in September as the change in season often brings out hatches of Blue Winged Olives.

The transition from summer to fall is usually easy to predict as by the middle of the month the first major cold front of the season passes through. For a few weeks after the cold front, tidbits of summer-like angling may still exist with the occasional grasshoppers, but by middle of September fall and its cool mornings, tree leaves full of color, and spunky trout are the name of the game on the Missouri River.

Streamflows are consistent below both Hauser and Holter Dams and water temperatures are very fish-friendly rarely rising above 65 degrees F and below 50 degrees F. Because the month begins feeling like summer and ends feeling like fall, September on the Missouri River is a fine sampling of all things that make the Missouri River so great—consistently good dry fly fishing; number-crunching nymphing; technical small dry fly fishing to rising trout; and streamer fishing for the big ones.

September fishing: what to expect

A typical day fly fishing the Missouri River in September can be different each day. It is also important to differentiate between early and late September, as weather and fishing conditions are vastly different from the beginning of the month to month’s end.

Similar to April, the fishing on the Missouri 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 Blue Winged Olive (BWOs) mayflies and aggressive, streamer-chasing brown trout increases.

Early September on the Missouri River can feel like a summer day. Fish can be very active before the sun is high in the sky. Early in the morning hatches of tricos can be thick and later in the day terrestrials may abound. The nights are cool and it may take a few hours for the morning chill to subside—September mornings are ideal sweatshirt weather.

If tricos have dwindled or the technical nature of fishing size 18 to 22 dry flies sounds daunting, fishing a tandem nymph rig or slowly dragging or swinging a streamer may entice more fish to strike than may rise to a small dry fly.

For anglers desiring to squeeze out some late summer terrestrial fishing, early September on the Missouri River is a good option. As the morning warms the bankside grasses, grasshoppers, ants, and beetles become more active. For anglers willing to commit to fishing terrestrials, early September can be some of 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 Missouri River downstream of Craig and north to Cascade is notorious for producing some big browns in late September.

Compared to early in the month, 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 worm, but on the rare occasion early risers are rewarded with a large brown trout on a well presented streamer.

By the middle of the month the substantial weed growth that occurred in August is usually gone—the longer, cooler nights having caused many weeds to die. With the river clear of floating weeds, drifting a fly becomes easier.

Where to find September trout on the Missouri River

Mayfly and caddis nymphs, midges, scuds, and sowbugs are active in the Missouri River year-round. In early September’s summer-like conditions of bright sunshine, trout are going to be found in subsurface holding lies. When 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.

Throughout September, but especially so in the first half of the month, trout will key in on terrestrials. Grasshoppers, ants, beetles, and crickets may land on the water. Look for trout rising to these land-based insects in any possible holding lie—pocket waters in-front, around, and behind rocks; drop-offs downstream of shelfs; in the heavy seams along the many large rocks; and along ambush-friendly bank structure.

As the middle of the month comes and water temperatures lower, hatches of Blue Winged Olive (BWOs) 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.

After the season’s 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

On the Missouri River, September doesn’t always mean the onset of fall—especially in the first half of the month. Because the weather in early September can still be warm, terrestrial fishing can be quite reliable and a few tricos may still hatch in the early morning hours. However, longtime Missouri River anglers patiently wait for the first cold front to move through and bring on hatches of Blue Winged Olives (BWOs) and October caddis.

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.

Trico mayflies may hatch in the early morning hours in the first half of September. Trout rarely feed on trico nymphs but will often rise to adults floating on the surface or “spent” adults—insects that have mated and are now dead. During a strong trico emergence, Missouri River trout group together in “pods” to rise to hatching tricos.

Terrestrials—grasshoppers, crickets, ants, beetles, spiders, and any other land-dwelling insect that may inadvertently find its way onto the surface–will be targeted by trout as well especially in the first half of September.

Early in September a few sporadic evening caddis hatches can occur on the Missouri River, especially on the shallow flats on the river around Craig.

Lastly, a few October caddis may be seen flying by late September. Hatches of October caddis are not predictable or even all that reliable, but for anglers wanting to catch a fish on a large dry fly, committing an entire day’s worth of fishing to a large dry fly may entice a few fish to strike. Most October caddis on the Missouri River are sizes 8 to 12.

Missouri River fly box for September

BWO dry flies sizes 16 to 22

BWO emergers sizes 16 to 20

BWO nymphs sizes 16 to 20

Trico nymphs in sizes 18 to 22

Trico duns/adults in sizes 18 to 22

Trico spinners in sizes 18 to 22

Caddis pupae size 14 to 18

Caddis CDC emergers size 14 to 18

Caddis dry flies with dark grey, black or brown bodies in size 14 to 18;

Grasshoppers, in tan or peacock, in sizes 4 to 14

Ants and beetles in sizes 12 to 18

October caddis in sizes 8 to 12

Scuds in sizes 10 through 20

Sowbugs in sizes 10 through 20

Midges, especially Zebra midges in black and olive, in sizes 16 through 22

Sculpin patterns in sizes 2 to 6

Streamers in olive, black or brown in sizes 2 to 6