Coho salmon and steelhead can be expected, as well as sea-run cutthroat. All species will need some rain to bring them into the rivers, so your success depends as much on what happened late in August as on what happens in September. The three-four miles below the hatchery will hold the most coho.
For the steelhead, use traditional patterns, such as Skunks, Green Butt Skunks, and Muddlers, in smaller sizes (6-10) and traditional tactics. Or use indicator tactics with Egg Flies or large nymphs. If the river is low, use a thin leader (8-pound/3X or less), approach and cast carefully, try to spot fish before casting to them, concentrate your efforts during the low-light times of day. During low water conditions, select a drab, small fly such as a Big Bird (great for indicator tactics) or a size 10-12 Muddler (traditional tactics).
When pursuing sea-runs, use bright size 8-10 streamers, Muddlers, Spruce flies, or Reverse Spiders. Retrieve them through slack water, backeddies, or slow water that is overhung by alders. You can also pick up sea-runs on nymphs, such as beadhead Pheasant Tails or Cased Caddis.
A good strategy is to float the river in pursuit of steelhead or coho. Fish a run, then as you drift to the next run you can pick up a trout rod and cast a streamer to slack water or skate an Elk Hair Caddis or Stimulator across the surface; sea-runs can slam your fly.
As salmon go on their redds near the end of the month, an Egg Fly drifted near the bottom can pick up cutts as well as steelhead. Egg Sucking Leeches are another good steelhead fly choice, as are Marabous in size 6-8.
For more on September tactics and flies, see the Rivers in General forecast