Flows on most rivers have dropped to their base flows. There are only a few relevant hatches this time of year but they can be important at times. Terrestrials are winding down but can still be productive on warmer days. The fall brown trout run is still a few weeks away on most waters but the browns are beginning to get colored up. In some fisheries browns are already beginning to move. Pay attention to water temperatures regardless of where you are fishing as this is a driving force during late September. As nights get longer and days get cooler trout are quickly transitioning from their summer lies. The fast riffle water where you were successful last week may now be devoid of trout. In general expect trout to slowly slide from faster water to slower water as temperatures cool.
Water temperatures are cooling and fish are moving into slightly slower water. Hatches are limited but terrestrials are still important. The majority of the early summer hatches are over and trout are often targeting terrestrials such as ants, beetles and hoppers. The larger fall baetis are still to come but the smaller psuedocleons (about a size 24 version of beatis mayfly) have had some strong emergences on some waters. Trout are also opportunistic when feeding subsurface and a variety of nymphs can be effective. If you are fishing early in the morning when the light is low streamer fishing can also be good. Pay attention to hydrographs and also to weather forecasts to be mindful of any strong thunderstorms or early fall rain events might dirty specific waters. Anytime thunderstorms or rain events are forecast make sure to look at flows in upper reaches to look for spikes which may mean a mud plug is on the way. This is mostly relevant to freestone fisheries such as the Yellowstone River.
Hatches are starting to wind down but are still relevant. Tricos hatches are largely over but a few are still lingering. The tiny psuedocleon version of the baetis are hatching on many waters. These are size 22 or 24 but can bring trout to the surface or at least get them feeding subsurface. There are also some sporadic larger mayflies that can entice fish to patrol and trying a larger size 10 or 12 gray wulff can sometimes bet the ticket.
Most of the area waters are fishing quite well now with the exception of some the higher elevation small mountain streams which are cooling fast. Spring creeks will have consistent water temps but are fairly technical this time of year. Tailwaters remain a solid early fall bet. Larger freestone rivers are an excellent option as water temperatures are at nearly ideal levels in late September on these waters.