Morning tricos and afternoon hoppers are still a factor throughout the Bitterroot; team your hopper with a size 16-18 Pheasant Tail or Copper John on a dropper. These bugs won't last much longer, though.
As the tricos and hoppers fade, blue-winged olives will dominate the river. Most BWO hatches will start about 10:00 a.m. and run until noon.
Mahogany duns are a major September hatch here. Look for rising trout near the margins of the stream. Most hatches are in quiet water, so avoid blind casting; pick your fish when it rises, then cast to it.
Another mid-month hatch is the big Hecuba mayflies (Timpanoga hecuba), which you can pretend is a size 12-14 green drake if you like. Regardless, a size 12-14 gray Parachute Hares Ear, Paradrake (see the Green Drake Paradrake for an example of the style), or similar fly will imitate it. Hatches are in the afternoon.
Evening caddis action can also be expected. Size 16-18 patterns work well, such as Elk Hair Caddis or Parachute Caddis, or try a size 14 Goddard Caddis. Some pale morning duns might be around, too. Both the PMDs and caddis are on the way out.
At the end of September, you may see some October caddis.
For more on September tactics and flies, see the Rivers in General forecast