September is a transition month in Montana. Fall can arrive early in the Rockies, but warm Indian summers are also possible. Most of the snow from the previous winter has melted out and river flows are low even on a big water year. Nights are becoming longer and temperatures start dropping. Summertime tourists thin out once kids go back to school and many locals set down their fly rods to take when archery season opens.
The beginning of September often feels like an extension of August with terrestrials ruling the day. Montana fishing in September is dominated by hoppers and terrestrials on years with when warm temperatures extend into the fall. Hoppers, ants, beetles and crickets always play an important roll in the early fall, especially on hot days. With the exception of the trico mayfly there are not many aquatic hatches in the bigger rivers. Trout are opportunistic and will often key in on larger subsurface food sources like crayfish and sculpins. With lower flows don't expect trout to always be along the banks. On many rivers like the Madison, Gallatin and Yellowstone trout begin moving into the main channels and riffles and are no longer along banks unless they hold swift and deep water. Paying careful attention to subtle depth and current changes can pay big dividends. Depressions in riffles and seams are great places to target trout in the fall. Some of the first storms of the fall begin moving in during the month of September. These rainy days always produce big fish. They are a great time to strip streamers or toss big ugly nymphs that imitate large food sources. Later in September these cloudy days begin targeting the fall baetis (blue winged olives). The fall baetis can produce terrific dry fly fishing on cloudy and drizzly days.