Last time I was at the Yak was by Cle Elum. Saw some huge salmon that I thought were steelhead. I heard Chinook swim through there all the time. My fiance caught a 30" Chinook on a rooster tail there last year, it almost broke the rod!
Zero steelhead there from what I hear:HMMM
I've heard people call large Yakima rainbows steelhead. I think they call them steelhead when they're over a certain size, maybe 22 or 24" but they're not true steelhead. There may be a small remnant of a real steelhead run, but if there is, it's small. There are kings in the Yak though.
Steelhead use to run in the Yakima and especially the Naches. They ended the hachery program several years back so very few if any remain.
Friends use to tell me stories of seeing Steelhead spawning in the creek that runs through Suntides golf course. About 10 years ago I saw a tribal biologist surveying the creek for Steelhead (I was actually golfing, i'm ashamed to admit), he said that only a few would return each year. That was 10 years ago so I'm sure that the "few" has now dimished to 0.
I fish the Yakima quite a bit and I have seen Kings in there lots of times. In fact two weekends ago, I saw a couple Kings roll. When the kings are in try an egg fly and you'd be surprised how well the trout will hit it.
Regarding steelhead, last year I fished a section of the Yakima above Lake Easton and accidentally caught a steelhead on an egg fly. I saw a second steelhead in the same section of river while I was there and several kings as well. I didn't know there was any in there either until then. Looks like there are still a few left.