October Fishing on the Boulder River

October weather, stream flows, and summary

The source for the Boulder River’s clear, fast flowing water is the Beartooth Mountain range near Yellowstone National Park. The river flows north out of the high country for nearly 80 miles to meet the Yellowstone River near Big Timber. Fly fishing the Boulder River in October is best suited for experienced anglers or for those anglers desiring solitude on a river transitioning from fall to winter. 

For nearly all of October the Boulder River is best suited for wading anglers, as low stream flows make floating difficult or even impossible. With average stream flows of around 200 cubic feet per second (cfs) rafts have a very difficult, if impossible, time navigating the river’s shallow rock gardens, shelfs, and sections of whitewater. Expert kayakers can enjoy floating the Boulder River but only very experienced kayakers should consider a trip. 

October on the Boulder River is a mix of wind, sunshine, rain, and snow. Averaging 5 days of measure-able rain or snow. The average daily high temperature is around 58 degrees F, but this varies from the beginning of the month to the end of the month, as the end of October can see average daily high temperatures in the high 40s degrees F. 

During October the two sections of the Boulder River—above and below Natural Bridge—experience different conditions. Above Natural Bridge the fast pocket water sections are exclusive to wading anglers and water temperatures often remain cold. Hatches of Blue Winged Olive (BWOs) can occur any day and the river is sheltered as it is backdropped by mountains and canyon walls. 

As the Boulder River tumbles under Natural Bridge the characteristics change. Hatches of BWOs are a possibility any day, but so is the chance of heavy winds. During October the lower sections of the Boulder River can be susceptible to prevailing westerly winds as the change of seasons comes to the valley. Fortunately, since most of the fishing in October is on foot, the wind is less of a factor than if fishing from a boat. 

Access points on the lower section of the Boulder River are spread far apart—the closest two being over five miles from each other. Wading anglers in October must be physically fit and prepared to cover ground while climbing and crawling over large rocks and boulders, plus much of the land on both sides of the Boulder River in the lower section is private property so anglers need to be very mindful of trespassing. 

For experienced anglers—both in skillset and in wading ability—the Boulder River in October is a solid option. With two distinct sections anglers can experience strong hatches of BWOs and solitude before winter puts an end to the fishing season. 

October fishing: what to expect

October on the Boulder River is typically a walk-and-wade fishery. With flows averaging around 200 cubic feet per second (cfs), very few floating crafts can navigate the pocket water and whitewater sections. Only experienced kayakers can expect to utilize a floating craft. But for anglers willing to hoof it and commit to cover some distance, October on the Boulder River can serve up solitude, potential for dry fly fishing with Blue Winged Olive (BWOs) mayflies, and trophy-sized brown trout. 

Anglers can expect plenty of 10 to 12-inch wild trout. The closer to the confluence with the Yellowstone River one fishes, chances increase for the possibility of a trophy-sized brown trout migrating up from the Yellowstone River to spawn. 

If streamer fishing for a big brown trout is the desired method, focus on the usual predator hangouts—deeper water near shallow water, hiding near structure, or along a cut bank. Many brown trout will begin to spawn in October. These spawning fish may be found on their redds on shallow gravel bars. Please avoid targeting spawning trout when they are encountered.

A typical day on the Boulder River in October can begin around noon. Because BWOs are the primary hatch and water temperatures in October rarely rise above 55 degrees, trout may not begin to feed until mid-day. 

Sun, rain, and snow can all happen on the same day. And many days the worst weather creates the best fishing conditions. BWOs tend to hatch in rainy or snowy weather. Brown trout, especially large brown trout, are inherently more sensitive to bright sunshine in the fall. Fishing in cold, rainy, or snowy weather may not be what you find as enjoyable but armed with quality gear and a good attitude it just might pay dividends. 

BWO dry flies in size 16 and 18 are the go-to choices if a hatch occurs. Fish these single dry flies on a 7.5- or 9-foot 5X leader. Use plenty of floatant on the fly and the tippet—fishing a dry fly on the fast water of the Boulder River often requires plenty of floatant. 

For streamer fishing, a size 4 to 8 heavy-sinking olive, black, or brown streamer fished on a floating line could entice aggressive brown trout. Drag, dead-drift, or slowly strip these large flies through the deeper pools and runs to entice a large trout to strike. 

If a hatch is minimal or non-existent, a tandem nymph right with a size 10 or 12 stonefly and a size 16 or 18 mayfly nymph fished on a 4X fluorocarbon tippet in soft water can find some fish. Target any slower water behind rocks or below drop-offs and shelfs and be sure to continue to mend your slackline. 

Where to find October trout on the Boulder River

Because stream flows during October on the Boulder River typically are below 200 cubic feet per second (cfs) trout are easy to find. Expect to find them in their usual Boulder River holding and feeding lies: 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 the many pools below a run of whitewater or large run of rapids.

If a strong hatch of Blue Winged Olive (BWOs) mayflies occurs, look for trout in slower currents and “softer water.” Soft water on the Boulder River can seem hard to find, but, target the slicks behind rocks, deep pools, or the runs after a stretch of whitewater. In October a BWO hatch can occur even if adult insects are not seen on the water. BWO nymphs are active throughout the river. In the soft water, fish a tandem nymph rig through the usual holding lies—drop-offs near shelfs, seams along bankside structure or large boulders, and through any deep pool.

Brown trout can be found in the usual predator hangouts—deeper water near shallow water, hiding near structure, or along a cut bank. Many brown trout will begin to spawn in October. These spawning fish may be found on their redds on shallow gravel bars. Please avoid targeting spawning trout when they are encountered.

Early in October, if sunshine is abundant and the air temperature hovers near 60 degrees F or higher, the slight possibility exists for a few trout to still target terrestrials. Trout will expend vital energy and take the risk from deep cover for a morsel as large a grasshopper, ant, or beetle. These large offerings may not pass by again that day. Fishing grasshoppers in October is a rarity but it does happen. 

Important October hatches

Compared to late June and July on the Boulder River, hatches in October pale in comparison to the stoneflies, Pale Morning Dun mayflies, caddis, and terrestrials of summer-time angling on the Boulder River. But the river is home to healthy populations of stoneflies, mayflies, and caddis and these nymphs are active year-round. These insects can be active in October even if they are not hatching, anglers fishing the Boulder River in October and desire dry fly fishing must hope for a strong emergence of Blue Winged Olive (BWOs) mayflies.

On the Boulder River, fall season BWOs 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. Overcast, rainy or snowy days see the strongest emergence and a day with light winds is ideal. 

October caddis can emerge on the Boulder River, but they are very sporadic. These large insects—size 8 to 10—flutter above the surface, occasionally landing on the surface to lay eggs. It is rare to see more than a few insects in a day of angling, but an opportunistic Boulder River trout may feed on these large dry flies. 

Boulder River fly box for October

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 nymphs in sizes 12 to 18

October caddis nymphs in sizes 8 and 10

October caddis size 8 and 10

Crayfish patterns sizes 2 to 8

Sculpin patterns sizes 2 to 6

Streamers in olive, black or brown sizes 2 to 6

Grasshoppers sizes 4 to 14