Re: Starting Rivers in Portland, Oregon?
Not many rivers open for trout right now, esp. on the west side.
Like @bigshayne says, you can indeed find small trout in the upper Clackamas. Also the Mollala R. and Salmon R. (near Mt. Hood). And countless streams in the coast range. None of these are open though.
Right now you can fish portions of the McKenzie and Middle Fork Willamette for trout, or go over to central OR for the Crooked, Metolius, Deschutes, or Fall R. I suppose of those four the Crooked could be more of what you might call a "starter" river.