September Fishing on the Montana Spring Creeks

September weather, stream flows, and summary
September fly fishing on Nelson’s, DePuy’s, and Armstrong’s spring creeks has a variety of opportunities. Whether it is fishing terrestrials and tricos during the first two weeks of the month to hatches of Blue Winged Olive (BWOs) mayflies in the second half of the month, September on the spring creeks can be exciting. As summer fades into fall in Paradise Valley, the mornings are crisp and the streamside trees come alive with color as leaves change, making the spring creeks a lovely place to spend a day fishing…plus there are plenty of trout to provide action. 

Weather, like the fishing, is varied. Throughout the month the daily high temperatures have a broad range—from 85 degrees F in early September to around 65 degrees F by month’s end. This temperature swing is the largest of the angling season. Measurable precipitation is a little higher than August, piling up to about 1.5”. The potential for the season’s first snowfall exists in September as average monthly snowfall is 0.3”. These variances in weather allow for a wide range of fishing on the spring creeks in September. 

September is a transition time for the spring creeks and anglers need to be aware that each day can be drastically different from the previous day. The change from summer to fall usually occurs in the middle of the month after the first major cold front of the fall season passes through. For a few weeks after the cold front, tidbits of summer-like angling may still exist, but by the middle of September, fall with its cool mornings and trees full of color arrives and is here to stay. 

September fishing: what to expect
For dry fly anglers September can be an exciting month. Early in September prospecting with grasshoppers, ants, and beetles can produce fish because a lack of hatches might mean trout are actively looking for terrestrials. Then, after the season’s first major cold front passes through in the middle of the month, the strong hatches of Blue Winged Olive (BWOs) mayflies return.

Regardless of early or late in September, a typical day fishing Nelson’s, DePuy’s, or Armstrong’s spring creeks often begins fishing a shallow-water, sight-fishing nymph rig with size 18 or 22 midge or mayfly nymphs. Long leaders of 12- to 15-feet are best, tapered to 5X or 6X fluorocarbon tippets. A tandem, shallow water rig on a 12- or 15-foot leader tapered to 5X or 6X fluorocarbon with a size 18 or 20 scud, sowbug or midge can produce fish. Most experienced spring creek anglers use small yarn or light pinch-on indicators and small size—6 to 12—split shot. Because most currents on the spring creeks run slower than larger rivers, less weight and shorter depths from fly to indicator are essential. Successful patterns include Sawyer Pheasant Tails, Zebra midges, and WD-40s.

Terrestrials can be prevalent early in September so it is a good idea to expect to prospect some with grasshoppers, ants, or beetles. Expect trout to be less eager to rise to terrestrials than on other rivers in September. The massive amount of subsurface food available to a spring creek trout does make it hard for a trout to want to rise for a dry fly, but for dedicated anglers fishing terrestrials on the spring creeks a few fish can still be fooled.

When hatches of BWOs are thick, simply look for rising trout. Fish a leader no less that 12-feet long and use size 6X supple tippet. Most fall season BWOs on the spring creeks are sizes 18 to 22. Dry fly anglers should be armed with supple tippets in sizes 5X and 6X, plenty of quality floatant that works well with CDC, and the ability to execute a reach cast.

Streamer anglers may find a few more trout willing to chase a streamer, but the nearby Yellowstone River—and its much more opportunistic trout—is much better suited for fishing streamer than the spring creeks. Choose size 4 to 8 sparsely tied, internally weighted streamers in black or olive. 

Where to find September trout Nelson’s, DePuy’s, and Armstrong’s spring creeks
Because September on the spring creeks is a mix of summer and fall, early September is going to fish a lot like summer and late-September is going to fish a lot like fall. Later in the month as hatches of Blue Winged Olive (BWOs) mayflies occur, look for fish in slow current seams, on shallow flats, a slow seam next to a fast seam, any shallow water adjacent to any deep water, eddy lines, and seams behind rocks or seams created by faster water being slowed down by slow water. 

Earlier in the month and with its lack of available hatches, most spring creek trout will be in deeper water. Because the creeks harbor so much food, any small change in depth or break in current can create a holding lie, which because of the abundance of food, can also be a feeding lie. Even if fish are not rising, it is highly likely they are still feeding. Target any change in current speed or change in depth, such as a bucket or small depression where the depth may be a foot or so deeper than the nearby bottom structure. 

On the spring creeks any three to six feet deep depression or bucket that is near a shallow flat or riffle can hold fish, and lots of them. Until a hatch occurs, most fish will be found in slower, deeper water and then as a hatch begins the fish will adjust their locations based on the available insects. 

Before the seasons first cold front passes—and hatches of BWOs regain their consistency—look for fish to be eating midges. Many of these fish can be seen because the water of the spring creeks is very clear. If fish are spotted and they appear to be active—shifting from side to side and opening their mouth—then a well-presented shallow-water tandem nymph rig may entice feeding fish in these subtle buckets potholes.

Important September hatches for Nelson’s, DePuy’s, and Armstrong’s spring creek
September on the Paradise Valley Spring Creeks starts out feeling like summer but ends up in full-on fall mode. Because the weather in early September can be warm, terrestrial fishing can be quite reliable throughout the month, hatches of small caddis can occur in the evenings, and tricos may hatch in the early morning hours as well. By the end of September, on Nelson’s, DePuy’s, and Armstrong’s spring creeks, it is all about hoping for overcast and rainy weather and strong hatches of Blue Winged Olive (BWOs) mayflies. 

Terrestrials—grasshoppers, crickets, ants, beetles, spiders, and any other land-dwelling insect that may inadvertently find its way onto the surface—can be targeted by trout as well, especially in the first half of September. 

Trico mayflies may hatch in the early morning hours in the first half of September. Trout rarely feed on trico nymphs but will often rise to adults floating on the surface or “spent” adults—insects that have mated and are now dead. 

A few caddis in sizes 18 to 22 can emerge in the evening hours and anglers can enjoy some solitude just before dark.  

Fall Blue Winged Olive (BWOs) mayflies can hatch after the first cold front passes through, typically around September 15th. 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 provide a few to several hours of dry fly fishing opportunities. 

Lastly, a few October caddis may be seen flying by late September. Hatches of October caddis are not predictable or even all that reliable, but for anglers wanting to catch a fish on a large dry fly, committing an entire day’s worth of fishing to a large dry fly may entice a fish to strike. Most October caddis on the spring creeks are size 10. 

Paradise Valley Spring Creeks fly box for September
Grasshoppers in tan, grey, or light yellow in sizes 10 through 16

Ants, black or cinnamon, in sizes 16 to 18

Beetles, hi-vis black, in sizes 16 to 20

Crickets in black in sizes 10 and 12

BWO dry flies size 18 to 22

BWO emergers size 18 to 22

BWO nymphs size 18 to 22

Caddis pupae size 16 to 20

Caddis CDC emergers size 16 to 20

Caddis dry flies with brown bodies in size 16 to 20

October caddis in size 10

Scuds in sizes 18 through 22

Sowbugs in sizes 18 through 22

Midges in sizes 18 through 22

Streamers in olive, black, or brown in sizes 2 to 6