I recently moved to western Washington. I am aware of the great steelhead rivers on the west side but I was wondering if there where the good rainbow or brown bearing waters are. I live near the mouth of the Chehalis River system. :dunno
The WDFW has some good info that will get you started. Try here first: http://www.wa.gov/wdfw/fish/prospects/county.htm
Also, you can look up stocking info (look up both fry plants as well as 'catchable' plants) and you can see if any browns are planted in your area. Lackamas lake is kinda down that way and gets a lot of browns - but it is a muddy bass lake for the most part.
Another good place to look is washinglakes.com for a county by county lake summary with some reports.