Here is a trip report from Tim Welsh who fished with his son on the Yakima River.
Just returned from a weekend trip with my son who is living in Seattle now.   We wanted to fish somewhere that was easy to get to and the Yakima River fit the bill as it is an easy 2 hour drive East of Seattle.  We hired a guide for a full day float out of Troutwater Fly Shop in Cle Elum.  This was on the recommendation of a friend who works with Washington Fish & Game – said that in Cle Elum, the two shops he would use are Troutwater or Red’s Fly Shop.  There are about half a dozen fly shops that offer guide service in Ellensburg which is just a little further away.  
We went with the full day float – and enjoyed a simple lunch on the river.   Met our guide (Max Saffle) at the shop at 7:30AM and were fishing by 8:15AM.   Covered about 9 miles of the river in his drift boat.  Fishing from the boat allowed us both to go through especially productive drifts multiple times (and as a result caught more fish!).  The Yakima River itself is 214 miles long from headwater to mouth – and it originates as an outflow from Keechelus Lake (which is the top of Snoqualmie Pass).  About half way through the float we had the Teanaway river joins the Yakima and we had some great action at that spot.  
Set ups were with 5 wt rods with a dropper rig and strike indicator.  Like many areas – a drag-free drift is key.  The takes were typically very subtle and you really could not set fast enough.  We started with a larger rubber legged stonefly (brown & black) – #10 with the dropper being a bead head nymph (silver bead with purple seemed to be especially productive).  Fish hit on both flies with the same frequency – but the “drift” was key.  Also switched up and had several rainbows on egg flies as the dropper (this time of year there are spawning salmon in the river) as well as a San Juan worm.  But again – the bead head nymphs seemed to be the best producer.  In the afternoon when it warmed up a bit – we eliminated the strike indicator and our top fly (a caddis pattern with rubber legs – size 12) was used.  Had some great top water hits.  With the boat – most of the casts didn’t exceed 40′.  Always working the seams, the foam lines and the deeper channels along the banks.  The water is gin-clear – and is almost all large gravel or boulders.   
We chose to fish the upper river vs the Yakima River Canyon as we wanted to fish a section with lower fishing pressure – this also allowed us to fish through sections multiple times without anyone waiting on us.  The river bank is lined with towering Ponderosa pines and cottonwood trees.  A beautiful float with color coming up on the foliage.  The river drops about 10′ per river mile and during our trip the flow was ~650 cfs.  In the summer (at peak) it can be 5X higher.  
If you ever get out to Washington – this is one Blue Ribbon river you should put on your list.  
Tight Lines!
Tim