Large numbers of coho can be expected to enter the Kalama in September, the bulk of them after the third week of the month. Look for water that has some flow, but is moving slower than steelhead water. Cast down-and-across, but retrieve the fly at the end of its swing. Coho are chasers and can follow your fly right to the rod tip, so don't quit your retrieve too soon.
Beginning September 1, the stretch of water below the first hatchery becomes fly-only (the "Beginner's Hole"). A popular coho fly on this river is Bumblebee. It's like a traditional steelhead fly with a body made from olive and black chenille. Some anglers swing a purple Egg Sucking Leech through the pools, but if you're snagging fish you should put on a floating line and a lighter fly.
The coho hot spots can be crowded, so get there early in the day (like, pre-dawn). If you find yourself surrounded by leaping, rolling coho, move on. They rarely bite when they're acting like that. Most of the coho action is in the lower five miles.
If you're looking for steelhead, tie on a size 6 or 8 black Woolly Worm with a red tail or an After Dinner Mint. The Kalama has a high south bank, so the river is shaded much of the day and fishing is not restricted to just the early morning and evening hours.
For more on September tactics and flies, see the Rivers in General forecast