As the weather turns cold, fishing becomes good for brown trout getting ready for the spawn. They are aggressive on streamers, big attractor dries, tiny nymphs and San Juan Worms.
Fall nymphers on the Green River do very well fishing size 16-22 pheasant tails, scuds, San Juan Worms and midge larva patterns below a small strike indicator. Add or subtract weight to keep flies down near the bottom. Achieving a natural drift is very important. Drop a 5X piece of tippet (2-5 feet long) from the hook bend of a cicada or attractor pattern and tie on a small nymph or scud imitation.
This allows you to fish both patterns at once and gives the fish a good choice. Patterns like bleached pheasant tail nymphs, chamois caddie, larva lace midge larvae and light pick scuds (with red heads) are all very good patterns for the two fly rig. Keep them a size 16 or smaller. Utah allows two flies or hooks on the same line.
The hatches are mostly very small baetis and midges but attractor flies like #10 Royal Wulffs or cicada/beetle patterns can produce some good action well into the fall.
The fish get used to certain things and will reject anything associated with danger. A dragging fly is one of the biggest fish repellents. Always keep lots of slack in the line. One S-curve is not enough. Wiggle a pile of slack onto the water so that line will feed out as the currents affect it. Many fly fishermen advocate a tight line for nymphing but on the Green River, forget it. Keep lots of slack out on the water and when a fish hits, set the hook quickly downstream, side arm, so the hook will be pulled back into the corner of the fishes mouth. This method of setting the hook also helps control lots of slack line better.
Try different fly patterns than those that are being used by everyone else. Try a small attractor fly such as a #18 Royal Wulff during a hatch and instead of a dry fly, use an emerger with a strike indicator.
This is streamer season and large trout can be caught with patterns such as wooly buggers, zonkers, Clousers Deep minnows, Micky Finns and baby trout bucktails. Fish them with a sink tip line and some weight to get them down. Have patience; streamers may not produce hot action but chances are that you'll get bigger than average trout on this method.
When fishing from a boat, cast close to shore and retrieve fairly rapidly in slow water and slower in fast water. Pause briefly after every three or four strips to let the fish catch up with the streamer. In extremely fast water, don't strip, just let the current swing the fly as you twitch the rod tip. If you are casting from shore, concentrate on fishing drop-offs and the wide flats below pools. Don't ignore the rocks out in the middle of fast water. Large trout hang in some of the hardest to fish places.
Use 3X-1X tippet material and keep the leaders short. Preferred rods are long (9-10 feet long) and 6-8 weight to handle long casts with heavy flies.
Terrestrials are not always big like the cicadas or hoppers. Ants and beetles are deadly for fish feeding along current edges, especially when the water level is rising. It's hard to track these small dark flies and a micro strike indicator (or small dry fly as a strike indicator) can make a big difference in being able to track the flies' progress.
Nymphers will often do well on San Juan Worms, cranefly larva, scuds as the water levels fluctuate. During high water releases fish will move into current seams behind dropoffs and back-eddies. Add weight and lengthen the leader to get deep along these drop-offs.
For more information on current conditions call any of the local fly shops.