September Fishing on the Jefferson River

September weather, stream flows, and summary

September on the Jefferson River means the river can be a viable option again. August and the heat of summer is gone, and hatches begin to return as water temperatures cool. The fishing follows suit with opportunities to fish terrestrials, head hunt during hatches of Blue Winged Olive (BWOs) mayflies, or chase aggressive brown trout with streamers. 

September weather features some 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 1.5” rain or snow. 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 600 cubic feet per second (cfs) but by month’s end stream flows actually rise above 900 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

During September fly fishing on the Jefferson River changes dramatically, with a distinct difference between early in the month compared to later in the month. It is more important to acknowledge the changes within the month of September.

In late September when fishing the Jefferson 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- or late- morning. 

Fly fishing the Jefferson River in early September behaves more like summer than fall. 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. 

For streamers, choose beadhead or conehead in white, olive or brown patterns in sizes 2 to 6. Be sure to have a few articulated patterns, such as a Circus Peanut or Sculpzilla. 

In early September, throughout the day, terrestrial fishing should improve on the Jefferson River as the air temperature warms and the sun warms the riverside grasses and trees. For anglers willing to commit to fishing terrestrials, early September can be the best few weeks of the year. 

Typically between September 15th and 20th, the first major cold front of the fall season passes through. When this occurs, brown trout begin to prepare for their spawn and become more aggressive. These larger trout actively seek out prey. Anglers seeking large brown trout may consider an early morning start as well and drag or slowly strip large streamers. With the colder water temperatures of late-September, the early bird rarely gets the worm as most trout are active later in the day. 

Where to find September trout on the Jefferson River

Because early September conditions on the Jefferson River are more like August, any type of hatch in early September is a rarity because the month’s aquatic insects tend to desire colder water temperatures. But because the river’s surrounding hay fields are still thick with grass, grasshoppers, ants, crickets, and beetles all provide food for trout. Nearly all dry fly fishing in early September will be with grasshoppers, ants, crickets, and beetles.

In early September’s summer-like conditions of bright sunshine, trout are most likely 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 browns 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

A variety of hatches occur on the Jefferson River in September. Terrestrials 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 find its way into the river could end up as trout food.

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. 

Jefferson River fly box for September

Grasshoppers sizes 4 to 14

Crickets in sizes 8 to 12

Ants in brown, cinnamon, and black sizes 12 to 20

Beetles in black sizes 10 to 18

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 4 to 10

Caddis pupae sizes 14 to 16

Caddis CDC emergers sizes 14 to 16

Caddis dry flies with dark grey, black or brown bodies sizes 14 and 16; 

October caddis size 8

Crayfish patterns sizes 2 to 8

Sculpin patterns sizes 2 to 6

Streamers in olive, black or brown sizes 2 to 6