From their website:
This area is a birdwatcher's paradise. Hawks, quail, pintails, mallards, coots, woodpeckers, kestrels, and widgeons are just the start of the list of birds that one might encounter in Oaks Bottom. The star of the show, though, is the Great blue heron, the official bird of the City of Portland. Oaks Bottom is one of the favorite places of a score of these impressive birds because of its proximity to one of the rookeries on Ross Island.
This is a little jewel in Portland. Living here for 10 years, I had no idea it existed until I came across it in one of my hiking books. It is an urban walk just off the 40-mile loop along the river, and is really near Oaks Park (Portland's Amusement park). The refuge is in a wetland where migratory birds stop and it is pretty cool to see how many different birds you can see. One of our favorites the day we were there were these amazing, majestic Blue Herons. Our guy was pretty little, but liked pointing out the things he saw. As he gets older, we will go back and try to figure out together which birds are which. This is going to be a favorite place I think.
Another bonus is it is in the Sellwood Neighborhood, so you can go watch birds and then walk to a nice lunch or coffee. It was great!