August Fishing on the Boulder River

August weather, stream flows, and summary

August on the Boulder River doesn’t occupy the crown jewel position that June and July do, but for anglers desiring solitude and walk-and-wade opportunities, the Boulder River in August may be a perfect match. For dry fly anglers the Boulder River in August has consistently good fishing with terrestrials—insects that live on land and may find themselves in the water—and hatches of caddis, tricos, and a few PMDs create just enough opportunity to keep the Boulder River in mind for an August fly fishing trip. 

Unlike June and July when rafts can navigate the Boulder River’s nearly 70-miles of boulders, pocket waters, rapids, and pools, stream flows in August are rarely above 300 cubic feet per second (cfs). But because rafts are rarely seen on the Boulder River in August, walking-and-wading anglers can enjoy a river and know that a floating craft is unlikely to come bobbing down the river. Walking-and-wading anglers on the Boulder River should be in good physical condition and expect to cover plenty of ground. The river bottom is made up of various sizes of boulders making wading an endeavor. As long as anglers are aware of the excessive physical nature of wading the Boulder River in August, the fishing can be quite good.

The river is best described in two sections—the upper and lower—sections. The upper section above Natural Bridge is characterized by fast pocket water. The Boulder River Road parallels the river in much of this section and a lot of access is available here as the river flows through Custer-Gallatin National Forest. 

The lower section begins downstream of Natural Bridge and is most often surrounded by hay pastures and cottonwoods. The river gets its name from this section because of the consistent riffles tumbling aggressively over boulders, then followed by short deep aquarium-like pools. On the bottom of many of these pools are large boulders. Almost 100% of the land bordering the lower section is private property making walk-and-wade access in August very difficult. 

August is one of the driest months of the year on the Boulder River with less than five days of measurable precipitation. Daily high temperatures during the first two weeks of August average well above 80 degrees F dropping to 70 degrees F by month’s end. Because the Boulder River is a freestone river dependent entirely on snowmelt runoff, stream flows gradually drop throughout the month, making walk-and-wade fishing that much more desirable. 

During the last two weeks of August, the nightly low temperatures are almost five degrees F cooler than earlier in the month. By the end of the third week of August, usually around August 25th, there is one less hour of daylight compared to earlier in the month. These longer, cooler nights result in a lower daily high temperature. These slight changes can make a big difference resulting in more actively feeding trout compared to earlier in the month. 

August fishing: what to expect

Fly fishing the Boulder River in August is vastly different than July. First and foremost, nearly all the fishing in August is done by walking-and-wading. Low stream flows, often below 300 cubic feet per second (cfs) mean only small kayaks tended by very experienced boaters can navigate the river’s pocket water and whitewater sections. 

Because the Boulder River has two distinct sections—upper and lower—the river has a minor split personality in August. The upper section is defined by steep mountain sides and an abundance of pine trees. The lower section flows through hayfields and cottonwoods. Anglers on the Boulder River in August may find more actively feeding trout above Natural Bridge, plus because much of the river flows through Custer-Gallatin National Forest, more of the river is accessible for walk-and-wade angling. 

PMD, caddis, and stonefly nymphs are active year-round, but during August on the Boulder River are much more active when water temperatures are below 65 degrees, so it is a good idea to get an early start. As air temperatures rise by mid-day, terrestrials become more active and more terrestrials may land in the river, enticing even more trout to be opportunistic feeders. 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. Consider fishing two dry flies. 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. 

A lesser known hatch—spruce moths—can occur in August. Hatching from the tops of coniferous trees, the larvae need hot weather to hatch. Why spruce moths desire water remains a mystery, but for anglers on the Boulder River many of these land-based moths land on the water and the river’s opportunistic trout rarely ignore a well-presented spruce moth. Spruce moths are usually mottled in grey and tan and can hatch as early as 9 AM, lasting the entire day. 

Where to find August trout on the Boulder River

Caddis, stonefly, and mayfly nymphs are active year-round on the Boulder River. Even with the lower stream flows of August, Boulder River trout remain opportunistic and remain in fast water holding- and feeding-lies. Look for trout to be 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.

On the upper river—above Natural Bridge—if a strong emergence of spruce moths occurs look for trout throughout all possible feeding lies—near bankside structure, seam lines between slow and fast water, shallow flats, riffle corners, and the heads of deep pools. 

It is also possible to find more trout in some of the deeper pools. With the low stream flows of August, these pools—often below a section of rapids or a long run—can hold an abundance of fish. As the sun rises and penetrates deeper into the water, trout may move back to safer holding lies, occasionally moving to feed on a nymph 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.

Trout in the Boulder River in August will remain active if water temperatures are below 66 degrees F. Once the water temperature rises beyond 66 degrees F, which is rare on the upper section of the Boulder River, trout will slow their feeding and seek out refuge in deeper, cooler water.

In early morning or late evening hours, trout may feed in fast and shallow water. As the sun rises trout may move to deeper water. In this deeper water they may feed on nymphs. Throughout the day as nymphs become less active and more terrestrials land on the water, a hungry trout may be willing to rise from some of the deeper runs to eat a large hopper. 

Important August hatches

August hatches on the Boulder River are sparse compared to June and July. However, that doesn’t mean the river’s wild trout cease feeding. Because the Boulder River is home to healthy populations of stoneflies, mayflies, and caddis, August on the Boulder River is more about stream flows and choosing the best times to fish rather than matching a specific hatch. 

Terrestrials—insects that live on land rather than hatch from the water—make-up a large portion of a Boulder River trout’s diet in August. Spruce moths, grasshoppers, crickets, ants, beetles, spiders, and any other land-dwelling insect that may inadvertently find its way onto the surface is likely to be eaten by a hungry trout on the Boulder River in August.

For aquatic insects, the Boulder River in August will see hatches of caddis throughout the day. Evening emergence will be the strongest. Spruce moths can also emerge in large numbers especially on the upper section of the river in Custer-Gallatin National Forest. 

In addition to spruce moths, trico mayflies will emerge in the early morning hours, followed by very sporadic hatches of PMD mayflies. 

Boulder River fly box for August

Grasshoppers in sizes 4 to 16

Ants; black, brown, or cinnamon in sizes 12 to 18

Beetles in sizes 10 to 18

Crickets in sizes 4 to 16

PMD nymphs sizes 12 to 18

PMD emergers sizes 12 to 18

PMD dry flies sizes 12 to 16

Spruce moths in grey or tan in sizes 12 to 16

Caddis pupae sizes 12 to 16

Caddis CDC emergers sizes 12 to 16

Caddis dry flies with dark grey, black or brown bodies in sizes 12 to 18

Trico dry flies in sizes 18 to 22