Welcome flytie! Western Washington rivers aren't known for great trout fishing so that maybe why you're having problems finding out about them. Plus, as others have pointed out, river trout fishing in Western Washington generally doesn't start until June (check regs). If a river or section of river gets runs of anadromous fish, you can pretty much bet that there won't be an awesome resident big trout fishery like you'd find in Montana. Plus, many of the fish people think are trout are actually baby steelhead or salmon so one must be very careful and be good at fish ID. Many of these rivers have runs of searun cutthroat trout that can be legally targeted at certain times of the year so you might check that out. Often, there is decent fishing for small (6"-8") resident trout above barriers like falls, etc. that the anadromous fish can't access and you may stumble upon a bigger fish. In your area, when they open, try just about any tributary to the North or South Fork of the Skykomish - the Foss, Beckler, Tye, Miller, etc. You could also go over to the Mountain Loop Hwy and fish the tribs of the South Fork of the Stilly or head up to the Snoqualmie and its tribs. You often will have these smaller creeks to yourself as many people prefer to fish other rivers with bigger fish like the Yakima.
In addition to the Gazetteer, other good resources are "Flyfisher's Guide to Washington" by Greg Thomas and for smaller streams, "Washington's Central Cascades Fishing Guide" by Dave Shorett. Dave also has books for the Olympics and the Rainer area. Start exploring and you may be surprised by what you find - but it won't be like what you'd find in Montana, Idaho or Colorado. Have fun!