If you're looking for trout on the Marys River, then I'd start by going upstream from Philomath. Bank access is limited due to private property, but you might be able to hike along the railroad. Biggest problem with the river is that the water is way too warm in the summer months, so you would only find trout where there are cooling waters entering, but that shouldn't be a problem this time of year. A friend & I fished it last summer and found the cutthroat to be on the small side, though I finally took home one that was decently over the minimum length.
Though it's not open for steelhead, it's conceivable you could encounter one or two. They'd be more likely headed towards the Tumtum River.
