Just for fun, I googled it. It looks like the river cats are farther upstream. WDFW website lists a couple of lakes in Clark County that are stocked with cats.
http://wdfw.wa.gov/fishing/washington/Species/1171/Regarding the flyfishing - reports are not too good down there. You can go north to the Kalama and Lewis Rivers, which are anadromatous fisheries - steelhead, salmon, cutthroat. The lower Kalama is fly only during the summer low water periods. Oregon has a lot of good similar type streams, mostly gear fishers.
The Deschutes near Maupin is a world class fly fishery in the late spring/early summer. The Salmonfly hatch around Memorial Day is supposed to be incredible but there's a lot of tubes and boats floating that weekend, so I'd check with the local shops about timing that hatch. In June, an Elk Hair Caddis in the evenings with an emerger dropper with often get you two fish on your line with one cast. Watch out for the bats that swarm at twilight!
Personally, I'd find a small stream off any of the rivers going into the Columbia and head upstream into the bush with a light 7 ft. rod. Sea run cutts hang in the upper sections of these tiny drainages in the late fall - early spring and are a blast to catch and release. Gorgeous native/wild fish.
