You should be able to find some SRC's (sea run cutthroat), at this time of the year. But like you alluded to, they are likely not very far upstream just yet. Try small'ish brown Rooster Tails with gold blades. They mimick crawdad fry, which the SRC's gorge on when they get into fresh water.
You could also use a torpedo bobber, 4-5' of leader, and a Borden Special fly.
Excerpt from Best Fishing in America:
Cutthroat Trout
"The Kilchis can offer pretty good fishing for native cutthroat trout, with a modest seasonal harvest limit available here and many coastal streams.
The trout season begins in late May, and at that time of year fishing can be good for resident fish.
While these fish are dispersed, spring trout fishing in Oregon Coast streams is often quite good in the upper reaches of rivers and creeks.
It might be quite worthwhile to consider a trip up Kilchis River Forest Road during the first few weeks of the season, while the water is cool.
Remember that all fishing is closed above the junction of the North and South forks and in all other tributaries. Check the regulations before fishing.
By July and August, searun cutthroat trout (which behave like steelhead trout but certainly not as large) are coming back into the tidewater sections and will begin working their way into the lower reaches of the free-flowing river section through the late summer.
More silvery and often larger than other cutthroat trout, after fattening up on shrimp and other saltwater foods, searuns can be aggressive toward flies and small lures.
In the stream section, cast into riffles and also in the tail-outs or edges of deeper pools.
Note that the Kilchis is among rivers closed to bait fishing until September, when fall salmon fishing gets underway.
So spring and summer trout fishing in the stream is all about artificial flies and lures. You can use bait for the searuns in tidewater, where trolling a worm or spinner or combination rig can be deadly, as can casting lures and flies."
Hope that helps. Tight lines and let us know how you do (and share some pics).
TD