September Fishing on the East Gallatin River

September weather, stream flows, and summary

For anglers designing an intimate, walking-and-wading experience on a small river with a variety of trout habitats, the East Gallatin River in September checks nearly every box. Even with its origins in the heavily populated Gallatin Valley, the East Gallatin River feels surprisingly remote. 

There is ample public access, but fishing from a boat is not allowed so this is a river best fished by anglers who enjoy being on foot. Anglers choosing to fish the East Gallatin River need to walk, and walk a lot, and walk while staying in the river to avoid trespassing. Landowners along the East Gallatin River are notorious for harassing anglers. If you choose to fish the East Gallatin River know the Montana Stream Access Law and abide by it. 

September features a little bit of summer, a little bit of fall, and in some years even a little bit of winter. Throughout the month the daily high temperatures have a broad range—from 80 degrees F in early September to around 60 degrees F by month’s end. Precipitation adds up to about 1.4”.  The potential for the season’s first snowfall exists, as the average monthly snowfall is 0.4’” Because the weather can be so varied in September, the fishing can as well and anglers need to be prepared for a diverse fishery and changing weather conditions. 

The first half of the month has average stream flows around 40 cubic feet per second (cfs) but by month’s end stream flows actually rise above 70 cfs because less water is diverted from the river to irrigate rancher’s hay fields for feed for livestock. In some years “hoot owl” restrictions are in place. This means fishing must stop after 2 pm as water temperatures become harmful to any caught fish. Fortunately, in most years by the second week of September stream flows have increased and the cooler weather of September causes water temperatures to drop. 

September fishing: what to expect

Because average weather patterns change throughout the month—early September surely feels like summer while late September feels like fall—there is a distinct difference in fishing options between early in the month compared to later in the month. 

In early September fish may be active before the sun is high overhead. Start the day fishing a dry fly because grasshoppers, ants, beetles, and spruce moths may still be prevalent. Nights in September are cool, taking a few hours for the morning chill to subside. A tandem nymph rig or slowly dragged or swung streamer may entice more fish to strike than might be willing to rise to a dry fly. Choose size 10 or 12 stonefly nymphs or size 16 or 18 mayfly nymphs, preferably beadheads. 

Hatches of trico mayflies may also be strong in early September. Hatching around sunrise these small mayflies can provide some exciting sight-fishing opportunities to selective trout. Choose size 18 or 20 parachute dry flies on a 12-foot leader tapered to supple 6X tippet. 

In late September when fishing the East Gallatin River, a later start is preferred. After September 15th nights are markedly longer and cooler than earlier in the month. Hatches of Blue Winged Olive (BWOs) mayflies can occur, but usually do not begin until mid-day. 

For streamers, choose beadhead or conehead in black, brown, or yellow/brown in sizes 6 to 10. Be sure to have a few articulated patterns, such as a small Sculpzilla.

In early September, throughout the day, terrestrial fishing should improve on the East Gallatin River as the air temperature rises and the terrestrials in riverside grass and hayfields become active. For anglers willing to commit to fishing terrestrials, early September can be the best few weeks of the year. 

Where to find September trout on the East Gallatin River

Early September on the East Gallatin River means warm daily high temperatures which means warm water temperatures. Because of this, trout are found in a variety of places near available food and can migrate throughout various river habitats on a daily basis. 

Because the river’s surrounding hay fields are still thick with grass, grasshoppers, ants, crickets, and beetles all provide food for trout. Most dry fly fishing in early September will be with grasshoppers, ants, crickets, and beetles.

If there is a strong emergence of trico mayflies, trout can be found along the banks, near structure or foam lines feeding on hatching or “spent” adults—insects that have mated and are now dead, floating lifeless in the current. 

In early September’s summer-like conditions, trout are going to be found in subsurface holding lies. If fishing tandem nymph rigs in early September, focus on the deeper water near shallow water, behind or in front of structure, or any place that can provide cover from predators or fast currents.

As the middle of the month comes and October gets closer, terrestrial fishing will dwindle but hatches of Blue Winged Olive (BWO) mayflies commence. When a strong hatch occurs, look for trout in slower currents and “softer water” such as the inside of river bends, seams behind rocks, and slower runs below riffles. With the season’s first cold front, brown trout also begin to grow aggressive prior to fall spawning. 

After the first cold front passes, look for brown trout in the usual predator hangouts—deeper water near shallow water, hiding near structure, or along a cut bank. Although most brown trout will spawn in October or November, a few brown trout may begin spawning in late September. They may be found on their redds on shallow gravel bars. Please avoid targeting spawning trout when they are encountered.

Important September hatches

If August feels long and drawn out on the East Gallatin River, September comes in with a bang. A variety of hatches occur on the East Gallatin River in September. Terrestrials and tricos dominate early in the month and Blue Winged Olive (BWOs) mayflies can be abundant in the second half of the month. Although not related to any hatching insect, brown trout begin to get aggressive in September as they prepare to spawn in October. 

In the first half of the month, the weather can still be warm and terrestrial fishing can be quite reliable. Grasshoppers, ants, beetles, spiders, crickets, and any other land-dwelling insect that may inadvertently finds its way into the river could end up as trout food.

In the early morning hours anglers can expect strong hatches of trico mayflies. These tiny black-bodied and white-winged insects often hatch at sunrise. Trout may feed on the adults, but they will feed on “spent” adults—hatched insects that have mated, died, and then float lifeless on the water. 

Fall BWOs can hatch after the first cold front passes through, typically around September 15th. A cool, slightly rainy or overcast day is ideal for a strong emergence of BWOs. These mayflies 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 can provide a few hours of dry fly fishing opportunities. 

Rounding out the hatches for September include a few caddis hatches in early September and the occasional early October caddis in late-September. 

East Gallatin River fly box for September

Grasshoppers sizes 6 to 14

Crickets in sizes 8 to 12

Ants in brown, cinnamon, and black sizes 16 to 20

Beetles in black sizes 10 to 18

Trico nymph in sizes 18 to 22

Trico dry fly flies in sizes 18 to 22

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 10 and 12

Caddis pupae sizes 14 to 16

Caddis CDC emergers sizes 14 to 16

Caddis dry flies in tan or light bodies sizes 14 and 16; 

October caddis size 8

Crayfish patterns sizes 6 or 8

Sculpin patterns sizes 6 or 8

Streamers in black, brown, or yellow/brown in sizes 6 to 10