August Fishing on the Stillwater River

August weather, stream flows, and summary

Even with flows that originate in the high country of the Beartooth Mountains, the Stillwater River in August experiences a slight change from its prime-time of July. As stream flows consistently drop throughout the month—starting at averages around 1,000 cubic feet per second (cfs) dropping to 500 cfs—anglers on the Stillwater River in August experience a river in its high-summer mode. 

As hatches of Pale Morning Dun (PMDs) mayflies have dwindled and caddis hatches are mostly in the evenings, the availability of aquatic insects on the Stillwater River is less frequent than in July. However, because the Stillwater River often remains cool and its currents are fast with ample habitat, trout still feed regularly. 

August is one of the driest months of the year on the Stillwater 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 Stillwater 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, especially in the river in Custer-Gallatin National Forest. 

During August, trout on the Stillwater River become more opportunistic, focusing on terrestrials—grasshoppers, ants, beetles, spruce moths and crickets—for the bulk of their diet. Spruce moths provide an exciting hatch in August on the Stillwater River. These land-based moths hatch from bankside pine trees. When an emergence is strong with many of them landing on the river and are eaten by hungry trout. 

A misconception about the Stillwater River in late August is that the fishing is poor compared to earlier in the month. If stream flows remain above 500 cfs floating anglers can still float and fish the lower sections of the river, usually below Absarokee. For the river’s entire run, wading anglers have ample riverbed to walk-and-wade without risking trespassing. However, access points are far apart from each other so walking-and-wading anglers must be prepared to walk a lot on uneven rocks and boulders. 

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. 

August fishing: what to expect

Unlike many larger freestone and tailwater rivers, water temperatures on the Stillwater River often remain cool throughout August. Although not in the prime 52 to 58-degree F range, rarely do they rise past 67 degrees F, which translates into actively feeding trout throughout the day. 

But even with the relatively cool water temperatures, trout still adjust their feeding habits by becoming sensitive to a bright sunshine. They may feed early or late in the day, retreat to cooler waters, or feed more in riffles—the fast current providing cover from overhead predators. All of this means anglers can expect slightly different fishing tactics than in July. 

Pale Morning Dun (PMDs) mayflies, caddis, and stonefly nymphs are active year-round, but during August on the Stillwater River are much more active when water temperatures are below 65 degrees, so it is a good idea to get an early start. Trico mayflies—tiny black-bodied mayflies—can hatch on the Stillwater River around sunrise. These mayflies are small, with most being size 18 to 22. For dry fly anglers looking for a challenge, these trico hatches can serve up some unique sight-fishing opportunities. 

As the sun gets higher on the horizon and air temperatures rise, terrestrials become more active and more terrestrials may land in the river. Choose a 7.5-foot or 9-foot leader tapered to 2X or 3X and fish a single grasshopper in size 10 or 12 and expect plenty of fish to seek it out. Grey, tan, or pink are favorite colors. It is also a good idea to drop a size 16 beadhead mayfly or caddis nymph off a grasshopper pattern.

Because grasshoppers, crickets, ants, spruce moths, and beetles become active later in the day, beginning the day with a two-fly weighted nymph rig makes the most sense. Choose two small weighted nymphs, such as a size 16 or 18 Pheasant Tail and a size 16 or 18 caddis pupae. 

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 Stillwater

The Stillwater River has a surprisingly high number of available food options for trout in August. Even with low stream flows and abundant sunshine, trout on the Stillwater River remain opportunistic and hold and feed in fast water 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.

As the sun rises and penetrates deeper into the water, trout move back to safer holding lies, occasionally moving to feed on a nymph floating by in the current. However, with the Stillwater River’s many riffles and shelves, trout often feed even in bright sunshine. 

If trout are not found in shallow riffles or runs, look for them 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. 

If water temperatures rise beyond 68 degrees F, which is rare, trout will slow their feeding and seek out refuge in deeper, cooler water. Plus, if there is a lack of food available, trout on the Stillwater River will lie in deeper holes and runs taking refuge from the bright sun to avoid overhead predators. In this deeper water they may feed on nymphs, but throughout the day as nymphs become less active and more terrestrials land on the water, a hungry trout may be willing to rise from the depths to eat a large grasshopper, cricket, or beetle. 

Important August hatches

When many Montana freestone rivers are experiencing dwindling hatches, the Stillwater River still has a variety of hatching insects. Pair that with cold water flowing from the Beartooth Mountains and there are plenty of opportunities for trout to feed, but it is the emergence of terrestrials that excite most Stillwater River veterans during August. 

In August, the emergence of the many species of terrestrials are the focus of anglers and trout on the Stillwater River. Terrestrials—insects that live the entirety of their life on land—provide a large portion of a Stillwater 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 Stillwater River in August. 

In addition to abundant terrestrials, trico mayflies will emerge in the early morning hours, followed by hatches of PMD mayflies. Caddis can hatch throughout the day, with the strongest emergence occurring in the evening hours. Tricos and caddis can hatch in abundance on the Stillwater River in August. 

Stillwater River fly box for August

Grasshoppers in sizes 4 to 16

Spruce moths in grey or tan in sizes 12 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

Caddis pupae sizes 12 to 16

Caddis CDC emergers sizes 12 to 16

Caddis dry flies with tan or brown bodies in sizes 12 to 18

Trico dry flies in sizes 18 to 22