During the last part of the month of September you can expect water temperatures to be down to the preferred range for trout throughout the park. Cooler air temperatures, especially in the morning, are typical. Hoppers remain a prime fly because they are easy to see and fish but you will start seeing more mayflies and caddis. Ants and beetles will also be good fly choice.
Lamar River, Slough Creek, and Soda Butte Creek
The Lamar Valley can become more challenging due to low water and a lot of pressure but it still can be a good choice. The crowds will be gone.
Yellowstone River
The Yellowstone will still be a very good choice. There will still be plenty little Yellow stoneflies. The river will have plenty insects, especially the Blue Winged Olives, spotted sedge and little sister caddis as well as more terrestrial activity.
Gardner River
All sections of the Gardner River will be good during the month of September. You will see hatches of Blue Winged Olives, two or three species of caddis and some little Yellow stoneflies. There will still be a lot of terrestrial insects around as well.
The Fall River and Bechler River
The Southwestern section of the park will continue to be a good option but decrease as the month gets near it end. The cool weather will have the trout very active and good numbers can be caught. Good hatches of mayflies and caddisflies will still be taking place.
The Snake River and Lewis River
The Southeaster corner of the park will still be another good option. The Lewis river channel can turn out some very nice trout. Both rivers will have good hatches and terrestrials will also be important.
The Gallatin River and Grayling Creek
The Northwestern Section of the park will be yet another good option. Both streams will have good hatches taking place. Of course, terrestrial imitations will also be effective. These two streams usually produce a lot of action and very good numbers of fish.
Madison River
The Madison River will be the most popular river in the park and rightly so. There will be a lot of anglers fishing the runs from Hebgen Lake. Good numbers of resident rainbows and brown trout will be caught throughout its entire length in the park. There will be good hatches of Blue-winged olives, Spotted sedge caddis, Little Sister caddis and White Miller caddis will still be present. Terrestrials including hoppers, ants and beetles will be popular flies.
Firehole River
The Firehole River will be a very good choice for the fly angler. It should turn out good numbers of trout throughout the day. White Miller Caddis will be plentiful along with some Spotted Sedges. Blue Winged Olives will be hatching good. Terrestrials will still be on the menu for the trout.