August weather, stream flows, and summary
August on the Gallatin River is unique. Because most well-known rivers in Montana are often fished from boats, the Gallatin River occupies a special niche for late summer fishing. For dry fly anglers it is also an ideal option for walk-and-wade fly fishing for wild trout. Appearing in full force are grasshoppers, ants, beetles, and crickets. Hatches of caddis are abundant on the Gallatin River in August as well. But, the most anticipated hatch of August on the Gallatin River is the emergence of spruce moths.
August is one of the driest months of the year on the Gallatin River with less than five days of measurable precipitation. In the Gallatin Canyon, 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 Gallatin River is a freestone river dependent entirely on snowmelt runoff, streamflows gradually drop throughout the month, making walk-and-wade fishing that much more desirable.
With the lack of a prolific hatch, trout on the Gallatin River become more opportunistic, focusing on terrestrials—grasshoppers, ants, beetles, and crickets—for the bulk of their diet. Spruce moths hatch in August on the Gallatin River. These land-based moths hatch from bankside pine trees. When an emergence is strong, hundreds of thousands of spruce moths flutter around, with many of them landing on the river.
A misconception about the Gallatin River in late August is that the fishing is poor compared to earlier in the month. 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 actively feeding trout.
August fishing: what to expect
The Gallatin River in August is very different than it is in July. Flows are considerably lower and hatches, with the exception of spruce moths, are much less prolific. Unlike many larger freestone and tailwater rivers, water temperatures on the Gallatin River in Gallatin Canyon remain cool. Although not in the prime 52 to 58-degree F range, rarely do they rise above the high 60s. But even with the relatively cool water temperatures, trout still adjust their feeding habits by becoming sensitive to an abundance of sunshine. With low flows and bright sunshine trout on the Gallatin River in August do most of their feeding early or late in the day or subsurface.
Anglers on the Gallatin River in August will primarily focus on the river in Gallatin Canyon and in Yellowstone National Park. One the river enters into Gallatin Valley, irrigation draws and suburban development affect streamflows, creating a poor combination for feeding trout—low streamflows and warm late-summer air temperatures.
PMD, caddis, and stonefly nymphs are active year-round, but during August on the Gallatin 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 Gallatin 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.
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. Choose two small weighted nymphs, such as a size 16 or 18 Pheasant Tail and a size 16 or 18 caddis pupae.
As the sun gets higher on the horizon and air temperatures rise, 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. 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.
Spruce moths can start their emergence in late July on the Gallatin River, but in many years early August sees the strongest hatches. Hatching from the tops of coniferous trees, the larvae need hot weather to hatch. When they do hatch the fluttering adults gravitate towards any nearby water. The desire for water remains a mystery, but for anglers on the Gallatin River it is a wonderful respite from the two-fly subsurface nymph rigs so common in August. 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 Gallatin
Caddis, stonefly, and mayfly nymphs are active year-round on the Gallatin River. Spruce moths can hatch with abundance in August. 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.
As the sun rises and penetrates deeper into the water or if the spruce moth hatch wanes, trout will 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 the Gallatin River in August will remain active if water temperatures are below 68 degrees F. Once the water temperature rises beyond 68 degrees F, which is rare, trout will slow their feeding and seek out refuge in deeper, cooler water. Because hatches are less consistent in August than in July, finding trout on the Gallatin River in August is about finding the food. If there is a lack of food available, trout on the Gallatin River will lie in deeper holes and runs taking refuge from the bright sun to avoid overhead predators.
The prolonged exposure to the bright summer sun also affects the behaviors of Gallatin River trout. In early morning or late evening hours, trout may feed in shallow water. As the sun rises trout will 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 the depths to eat a large hopper.
Important August hatches
In the Gallatin Canyon south of Bozeman to south of Big Sky and into Yellowstone National Park, the spruce moth hatch takes the cake on this mountain freestone. In addition to spruce moths, trico mayflies will emerge in the early morning hours, followed by sporadic 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 Gallatin River in August.
In August, emergence of the many species of terrestrials are the focus of anglers and trout on the Gallatin River. Terrestrials—insects that live the entirety of their life on land—provide a large portion of a Gallatin 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 Gallatin River in August.
Gallatin River fly box for August
Spruce moths in grey or tan in sizes 12 to 16
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
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