August Fishing on the East Gallatin River

August weather, stream flows, and summary

Despite its origins in an urban environment—the East Gallatin River begins in the city limits of Bozeman—this small freestone is a local favorite. Fishing from boats is not allowed on the East Gallatin, so this is a walking-and-wading river only and in August the low stream flows allow anglers to cover plenty of water. 

Stream flows on the East Gallatin River during August rarely are above 50 cubic feet per second (cfs) and the average daytime high temperature rarely dips below 80 degrees. These low flows combined with high air temperatures and bright sunshine, often cause the waters of the East Gallatin River to be too warm for actively feeding trout. 

During August on the East Gallatin River there are less than three 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 79 degrees F by month’s end. 

In early August as the peak daily water temperature climbs past 68 degrees, the majority of the East Gallatin River’s hatches subside. With the lack of a prolific hatch, trout on the East Gallatin River become more opportunistic, focusing on terrestrials—grasshoppers, ants, beetles, and crickets—for the bulk of their diet. 

August fishing: what to expect

In years with average or above average snowpack, early August on the East Gallatin River can see fish-friendly stream flows and hatches of Pale Morning Dun (PMDs) mayflies, caddis, and trico mayflies. In years with below average snowpack, lower than average stream flows will hinder hatches and fishing becomes entirely subsurface or dependent on available terrestrials. 

But the consistent factor in August is the East Gallatin River is ideal for anglers who enjoy strapping on a backpack or loading up the vest and venturing out to cover a lot of water. Because stream flows are low, anglers can fish for long distances and remain below the mean highwater mark, reducing the risk of trespassing. If this is your plan, it is best to plan to start very early or fish late into the evening.

A typical day in August usually begins early, often before daybreak. PMD and caddis nymphs are active year-round, but during August on the East Gallatin River are only active when water temperatures are below 65 degrees. Trico mayflies can hatch around the coolest time of the day, typically around sunrise. These mayflies are small, with most being size 18 to 22. 

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.

Surrounded by acres of farms and grasslands, ample habitat exists for grasshoppers, crickets, ants, and beetles. These terrestrials provide plenty of food for hungry trout and a wide range of opportunities for anglers committed to fishing dry flies. However, the challenge on the East Gallatin River is always the low stream flows combined with warm air temperatures creating water temperatures that tick close-to or rise-above 70 degrees F. 

If water temperatures stay below 68 degrees, fishing is a viable option. Consider fishing two dry flies simultaneously, yes two dry flies at the same time. 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 East Gallatin

Because stream flows are low in August, trout do not migrate throughout various lies, but they will remain active if water temperatures remain below 68 degrees F. Daily high-water temperatures often rise above 70 degrees causing trout to slow their feeding habits. Once the water temperature rises beyond 68 degrees F trout will slow their feeding and seek out refuge in deeper, cooler water. Because hatches are less consistent in August than in July, you need to focus on finding cool water that is conducive to feeding trout. 

Caddis, a few small stonefly species, and mayfly nymphs are active year-round on the East Gallatin 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. Until a hatch occurs or terrestrials are plenty, focus on deeper water near shallow water, behind or in front of structure, or any place that provides cover from predators or bright sunlight. 

Because the prevalence of bright sunshine affects the behaviors of trout, the early morning or late evening hours may be the only time fish feed in water less than four feet deep. 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, trout may rise from the depths to eat a large hopper, ant, beetle, or cricket. Banks with deeper water or structure that provide cover are ideal places for opportunistic trout. 

Important August hatches

Because stream flows on the East Gallatin River in August are low—often below 50 cubic feet per second (cfs)—hatches are minimal compared to other months. Trico mayflies can emerge in the early morning hours, followed by sporadic hatches of Pale Morning Dun (PMD) mayflies. PMDs will most likely only hatch early in the month. Some small caddis can hatch throughout the day, but most emergences occurring in the evening hours during sunset. 

On the East Gallatin River in August the main source of food for trout are the abundant terrestrials. But for trout to feed actively on these land-based insects, water temperatures need to remain below 68 degrees F. Terrestrials—insects that live the entirety of their life on land—provide a large portion of an East Gallatin River trout’s diet in August. Grasshoppers, crickets, ants, beetles, spiders, and any other land-dwelling insect that my inadvertently find its way onto the surface, fall under the terrestrial moniker. 

East Gallatin River fly box for August

Grasshoppers in sizes 6 to 16

Ants; black, brown, or cinnamon in sizes 14 to 18

Beetles in sizes 12 to 18

Crickets in sizes 8 to 16

PMD nymphs sizes 14 to 18

PMD emergers sizes 14 to 18

PMD dry flies sizes 14 to 16

Caddis pupae sizes 14 to 16

Caddis CDC emergers sizes 14 to 16

Caddis dry flies with light gray or tan bodies in sizes 14 to 18

Trico dry flies in sizes 18 to 22

Crayfish patterns in sizes 6 to 8

Sculpin patterns in sizes 6 to 6

Streamers in white, black, or brown in sizes 6 to 10