Most of the year-round streams in the regulations are worth exploring when the flows are cooperating. That’s infrequent this time of year, especially at lower altitudes. I use the gauge on the Little Luckiamute to determine if I can reasonably fish the Yamhill and Tualatin systems. If it’s under 300 CFS and falling you can find trout. Ideal is below 100 CFS. This would be a good week to get out there.
http://levels.wkcc.org/?f=8w3
In addition to those that have already been mentioned, Fifteenmile Creek and all of its tributaries are now year round and sit outside the Mt. Hood rain shadow.
Just to help you set expectations, the reason there are so many year round trout streams on the west side is due to the amount of private property. It takes a lot of patience and scouting to find public access, and I’d recommend having multiple streams on your list for one day so that you can fish enough holes to make it worthwhile. Most of these streams are not designated as “navigable,” so accessing at a bridge and wading below the high water line won’t cut it. If you’ve ever spent time in the coast range foothills, you know these property owners aren’t folks you want to tangle with. They’ve got more bullets than brain cells.
Small streams are also just hard to fish in winter, they’re often too high and muddy to approach and even when they’re not that cold, high gradient water isn’t super appealing to trout. I love small stream fishing too, but I’m happy to get 1-2 decent fish (10+ inches) fishing these creeks in the off-season.
If you expand your radius to 2-2.5 hours from Portland, you’ll start to find higher quality year round trout streams with a lot more public access.