Don't rule out spider and revers spider flies when trying for SRCs. Or even stimulators. Many smaller rivers and streams have SRCs in them, you just need to know when. Generally, they move into streams July through November. That depends on the stream though. On average, they tend to move into the streams late August through October. You might be able to find many though along the mouth of the Columbia as this is where most of them would be in the Summer.

Don't rule out the South Sound if you have the time. http://www.washingtonflyfishing.com/columns/src.htm