August Fishing on the Bighorn River

August weather, stream flows, and summary

Located in southeastern Montana, about 90 minutes from Billings and about 75 minutes from Sheridan, Wyoming, the Bighorn River flows for nearly 100 miles before it joins the Yellowstone River. Fly anglers are primarily interested in the river’s first thirty miles of clear, cold flowing water downstream of Yellowtail Dam. The dam creates the Bighorn Canyon National Recreation area…and also creates a world-class trout fishery, home to around 4,000 trout per mile. Resembling a large spring creek, fly fishing the Bighorn River in August serves up opportunities for matching the hatch, action on subsurface nymph rigs, and casting terrestrial patterns in hopes an opportunistic trout will rise. 

The home base for the river’s services—fly shops, shuttles, and most lodges—is the small fishing-centric town of Fort Smith. The river also flows through the Crow Indian Reservation so riverside development is minimal. If the desire to fly fish a river that hasn’t seen growth and development, the Bighorn River in August may fit the bill. However, because there is approximately thirty miles of cold water and good trout habitat, most of the angling use is concentrated on the upper river. Other anglers and boats will be seen, but very few houses or development exists on the Bighorn River. 

Stream flows in August are rarely above 3,000 cubic feet per second (cfs) and in most years average around 2,500 cfs. Because of these low flows, wading anglers relish in the myriad of opportunities on the Bighorn River in August. Water temperatures rarely rise above 65 degrees, especially on the river from Afterbay Access to Bighorn Access Site. Below Bighorn Access downstream to Mallards Landing can see water temperatures rise above 65 degrees on a daily basis. 

During August on the Bighorn River there are less than four days with measurable precipitation, averaging slightly less precipitation than July. Daily high temperatures during the first two weeks of August average well above 85 degrees F dropping to 80 degrees F by month’s end. The potential exists for daytime highs in the mid to upper 90 degrees F as well. 

August on the Bighorn River is surprisingly diverse. Whether it is nymphing with two-fly weighted flies or targeting trout sipping on trico mayflies or prospecting with hoppers in the midday sunshine, floating and walking-and-wading anglers will find action on the Bighorn River when other rivers in Montana may be too warm to too low for consistently good fishing. 

August fishing: what to expect

Because stream flows on the Bighorn River in August are relatively consistent, anglers on the Bighorn River can expect hatches of tan and black caddis, tricos, a few Pale Morning Duns (PMDs), and a variety of terrestrials. In most years, the predictability of the fly fishing action in August is related to early summer stream flows. Simply put: if stream flows in June and July are consistent with historical averages and patterns—peaking in June at levels above 7,000 cubic feet per second (cfs) then gradually decreasing to around 3,000 cfs by August 1st—then the Bighorn River in August is the most consistent late-summer fishery in Montana. 

For the best action, a typical day in August usually begins early, often before daybreak. Pale Morning Dun (PMDs) mayfly and caddis nymphs are active year-round, but during August on the Bighorn River are most active when water temperatures are around 65 degrees F. Black caddis dominate the hatch charts in August, while tan caddis start strong in July and usually trickle out by the end of August. 

Fishing hatches of black caddis on the Bighorn River is often best in the evening hours, with the strongest hatches occurring during the hours before and after sunset. For dry fly enthusiasts, fishing a black caddis hatch on the Bighorn River often allows anglers to experience some solitude on an otherwise busy river---many of the river’s casual anglers are off the water well before sunset. 

For a black caddis hatch, choose dark-bodied caddis in size 18. Patterns with CDC are ideal, so be prepared with a powder floatant and practice your reach cast. Popular patterns are Bubbleback Caddis, CDC Black Caddis, and the Poodle Sniffer. 

Trico mayflies—tiny black-bodied mayflies—can hatch on the Bighorn River around sunrise. If trico hatches are thick, Bighorn River trout may group together in “pods” and feed on hatching, adult, or spent—mated and dead—tricos. These mayflies are small, with most being size 18 to 22. For dry fly anglers looking for a challenge, these Bighorn River trico hatches can serve up some unique sight-fishing opportunities. 

Mayfly and caddis nymphs, midge larvae, scuds, and sowbugs are active year-round and these are the go-to choices for prospecting with a two-fly weighted nymph. Because grasshoppers, crickets, ants, and beetles become active later in the day, beginning the day with a two-fly weighted nymph rig makes the most sense. As the sun gets higher on the horizon and air temperatures rise, terrestrials become more active. If the wind begins to blow or ranchers and farmers harvest the surrounding fields, more terrestrials land in the river enticing even more trout to be opportunistic feeders. 

These terrestrials are the reason many anglers fish the Bighorn River in August. Flanked by acres of farms and grasslands, ample habitat exists for grasshoppers, crickets, ants, and beetles. Blowing into the river, these terrestrials provide plenty of food for hungry trout and a wide range of opportunities for anglers committed to fishing dry flies. A tactic gaining popularity in recent years is to fish two dry flies simultaneously. Choose a grasshopper in size 8 to 12 and an ant or beetle in size 14 to 18. Similar to fishing a two-fly weighted nymph rig, fishing two dry flies increases the chances for success. 

Where to find August trout on the Bighorn River

If hatches of black or tan caddis are thick, look for Bighorn River trout to move from traditional late summer holding lies into fast-water feeding lies. These are riffles, seams along riffles, shallow flats, or the tailouts of long runs or flats. 

As tricos hatch in the morning, look for Bighorn River trout to feed along riffle seams or seams originating from bank-side structure. As adult tricos hatch and mate, then die—dead tricos are called “spinners”—spinners may accumulate in eddys or foam lines. Trout may feed on spent tricos well into mid-day. 

Caddis and mayfly nymphs, midges, scuds, and sowbugs are active year-round on the Bighorn River. In August, as the sun rises and penetrates deeper into the water, trout may stay in holding lies, occasionally moving to feed on a nymph, midge larvae, scud, or sowbug floating by in the current. These trout can be found in classic subsurface lies: deeper water near shallow water, behind or in front of structure, or any place that provides cover from predators or bright sunlight. As the sun rises trout will move to deeper water. In this deeper water they may feed on nymphs, midge larvae, scuds, or sowbugs. 

Throughout the day as subsurface insect actively becomes less active and more terrestrials land on the water, a hungry trout may be willing to rise from the depths to eat a large hopper. Banks with deeper water, structure that provides cover, and many of the Bighorn River’s long flats and riffle corners are ideal places for opportunistic trout. 

Important August hatches

Hatches on the Bighorn River in August are not as abundant compared to June, but compared to some of Montana’s other freestones, the hatches are thick. Black and tan caddis, tricos, and a few Pale Morning Dun (PMDs) mayflies hatch in August. Terrestrials can also be thick as well. On any given day during August anglers can experience a strong emergence of any of these trout foods. 

During August black caddis hatch primarily in the evenings on the Bighorn River. Tan caddis also hatch in August but dwindle later in the month. Tan caddis can hatch throughout the day, while black caddis tend to hatch in the morning or evening hours. 

Trico mayflies hatch in the early morning hours. Trout often rise to adult tricos floating on the surface or “spent” adults—insects that have mated and are now dead. Tricos usually hatch along bankside structure or from many of the Bighorn River’s riffle corners. During a strong trico emergence, Bighorn River trout group together in “pods” to rise to hatching tricos. 

Terrestrials—insects that live the entirety of their life on land—provide a large portion of a Bighorn River trout’s diet in August. 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. 

PMDs, if they hatch in August, hatch exclusively in the morning hours. Often concurrently with tricos, it is important to know the difference. Tricos are black-bodied and have white-wings while PMDs are light-bodied and have pale or grayish wings. Additionally, trout may feed on PMDs spinners—adults that have mated and are “spent”—floating lifeless on the water. 

Bighorn River fly box for August

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 with black or tan bodies in size 16 to 22;

PMD nymphs in sizes 16 and 18

PMD emergers in sizes 16 and 18

PMD dry flies in sizes 16 and 18

PMD spinners in sizes 16 and 18

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

Ants and beetles in sizes 12 to 18

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