September Fishing on the Bighorn River

September weather, stream flows, and summary

For those that know the Bighorn River intimately, September is often the most challenging month of fly fishing on the Bighorn River. However, this varies from year to year and because the reputation of September fly fishing on the Bighorn River is lower than earlier months, September is the month for solitude on this southeastern Montana tailwater. Hatches of tricos and black caddis can be thick and fly fishing with terrestrials can still be very good. 

The river’s flows are regulated by Yellowtail Dam. In most years stream flows remain around 2,500 cubic feet per second (cfs) from early August through September, the potential for expansive growth of weeds is high during September. The amount of weed growth varies from year to year and the predictability is related to many factors, therefore guessing how it will affect the fishing in any given year is impossible. 

Even with September fly fishing on the Bighorn River a product of variability, the pleasant weather and lack of crowds compared to June and July make it an enjoyable experience. Early September feels more like summer, yet late September—with the slight possibility of snow—can feel like fall or even winter. Around Labor Day grasshoppers and black caddis can be plentiful, by the middle of September the potential for Blue Winged Olive (BWO) mayflies increases, and by month’s end prospecting with large streamers could produce a trophy-sized brown trout. All of this as the river’s cottonwood trees begin to come ablaze in fall colors. 

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 possibly snow. Measurable precipitation accumulates at around 1”. The potential for the season’s first snowfall exists in September as average monthly snowfall is 0.1”. These predictable weather changes are good news to anglers fly fishing the Bighorn River in September as the change in season often reduces the abundance of weeds in the river and brings out hatches of Blue Winged Olive mayflies.

September may be less than what Bighorn River loyalists are accustomed to, but for anglers who enjoy fishing during the transition from summer to fall, the Bighorn River is a fine river on which to enjoy the change of the seasons. 

September fishing: what to expect

It is very 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. Because the growth of aquatic weeds and vegetation in the Bighorn River in September is a fact of life, anglers must prepare to adjust fishing tactics or techniques. 

Deep water nymphing with two-fly nymph rigs is challenging for most of September, as the weeds make getting a drift without weeds clinging to the flies difficult. Additionally, many of the deeper runs ideally suited for using nymphs below and indicator are thick with aquatic vegetation. 

If tandem nymph rigs can be fished effectively, scuds, sowbugs, caddis pupae, and mayfly nymphs are the go-to patterns. Choose patterns in size 18 to 22, fished on a 9-foot 4X or 5X leader with fluorocarbon tippet. 

In early September the potential for hatches of tricos in the morning and caddis in the evening is always high. Plus, the possibility of consistently good terrestrial fishing exists. 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. 

The determining factor for being able to successfully fish the hatches or fish streamers in September is often related to the amount of weeds in the river. Up to date reports and local knowledge are crucial to having success in September. There is simply no way to predict how much, or how little, weeds may exist from one year to the next. 

Early September on the Bighorn 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, in the afternoon, terrestrials may abound, and in the evenings black caddis can emerge. For anglers willing to commit to fishing terrestrials, early September can be some of the best few weeks of the year.

By the middle of the month the substantial weed growth is usually gone—the longer, cooler nights having caused many weeds to die. With the river clear of floating weeds, fishing streamers or two fly weighted nymphs becomes easier. 

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. With less weeds and aquatic vegetation in the river, anglers can easily strip, drag, or dead-drift streamer patterns. Now begins the time when anglers committed to fishing streamers can find some of the largest brown trout of the year. 

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. 

Where to find September trout on the Bighorn River

Mayfly and caddis nymphs, midges, scuds, and sowbugs are active in the Bighorn River year-round. In early September’s summer-like conditions of bright sunshine and thick aquatic weeds, unless there is a significant hatch or a large amount of terrestrials on the water, trout are going to be found in subsurface holding lies. 

Although weed growth is often thick, making dead drifting two fly rigs challenging, fishing tandem nymph rigs in early September can still reap some rewards. 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. Be prepared to regularly clean the flies. 

However, even with thick weed growth, throughout September, trout will rise to terrestrials. Grasshoppers, ants, beetles, and crickets may land on the water in large numbers on any given day. But on a day when the wind is blowing from the west, be prepared for many of the Bighorn River’s trout to actively pursue terrestrials. The river flows north to south, so a westerly wind blows plenty of food onto the water. 

When this occurs, look for trout rising to these land-based insects in any possible holding lie—in  the many long riffles; drop-offs downstream of shelfs; along ambush-friendly bank structure; throughout many of the long shallow runs; and the tailouts of deep pools.  

As the middle of the month comes and water temperatures lower, hatches of Blue Winged Olive (BWOs) mayflies can be thick. When a strong hatch occurs, look for trout in slower currents and “softer water” such as the inside of river bends, seams along riffles or along bank-side structure, and slower runs below riffles. 

After the season’s first cold front passes—and many of the weeds finally die-off and float away—look for brown trout in 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 in southeastern Montana can feel more like summer than it can in western Montana. Because the weather in early September can be warm, terrestrial fishing can be quite reliable throughout the month, black caddis can be thick in the first few weeks of the month, and tricos may hatch in the early morning hours as well. 

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, Bighorn River trout may 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. 

Black caddis in sizes 18 to 22 can emerge in the evening hours. Even with the prevalence of weeds, anglers desiring some consistently good dry fly fishing can find fish feeding on black caddis during the first few weeks of the month. 

Fall Blue Winged Olive (BWOs) mayflies 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. 

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 Bighorn River are sizes 8 to 12. 

Bighorn 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 16 to 22

Caddis CDC emergers size 16 to 22

Caddis dry flies—black bodied--size 16 to 22

Grasshopper, 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 yellow/brown, olive, black,or brown in sizes 2 to 6