Blackfoot:Nymphing and streamer fishing will be a good bet. Stonefly nymphs followed by a prince or sjw should be all you need. Slow trolling streamers on a tip should connect with some fish too.
Fly Patterns:Girdle bugs, db Stones. SJWs, general beadheads, Glo bugs, buggers / streamers
Lower Clark Fork: Stable flows and fishing well. A skwala foamy with a deep dropper in the prince /g-bug/ sjw variety will be a good way to start. Tandem nymph rigs through the deep seems and deep back eddies will find fish. Trolling streamers on a tip should find fish too and is more fun than starring at a bobber if you can’t get them to eat the dry. BWOs and March browns should appear in the afternoon so keep your eyes open for risers. A skwala trailed by a p-tail cripple or purple haze chute will work for the rising fish.
Fly Patterns:Girdle bugs, db Stones, prince nymphs, fb p-tails, SJWs, Glo bugs, buggers / streamers, purple haze chutes, p-tail cripples, foam skwala stuff
Flathead:The main stem is a little high but fishable. They are ramping flows out of the South Fork so head up to Blankenship or the forks to get more manageable flows. If you are fishing the mainstem, look for soft eddies and slower runs. Tandem nymphs fished deep and slow will be your best bet.
Fly Patterns: Pats rubber legs / girdle bugs, db epoxy stones, 20 inchers, general beadheads, pink stuff, glo bugs, sj worms in pink or red, buggers, streamers, foam skwala variations, midges, bwos.
Swan: The upper is closed through the 3rd saturday in May. Streamers and nymphs fished slow and deep will be your best bet for the lower river and bay.
Fly Patterns: Pats rubber legs / girdle bugs, db epoxy stones, 20 inchers, general beadheads, pink stuff, glo bugs, sj worms in pink or red, buggers, streamers