Despite the fact that Portland, OR is justly famous for its parks (or greenspaces in yuppie speak), and the fact that it is at the junction of two major rivers, there is not actually that many places to go for a natural riverside experience in Portland.
Oaks Bottom, also known as Oaks Bottom Wildlife Refuge, is one of the few places in Portland proper that it is possible to see the Willamette River in its natural glory, or anything close to it. At 165 acres (depending on how high the river is), it provides a succint view of the wetland environment that used to cover the low lying areas of the Eastside of Portland.
The park lies just across from Ross Island and immediatly north of Oaks Amusement Park and the Sellwood Bridge. Although it appears to be a natural environment, it was actually created as a park in the 1950s when a contractor received the rights to dump construction waste in exchange for building a park when he was done. Thus, Oaks Bottom came into existence.
The park has a very large lagoon in the middle of it, and during the wet months (which in Oregon, as stereotype correctly states, is between October and May) even the meadows and forests are often under a few inches of water. The trails can get pretty muddy too. Of course, as the name suggests, this is a wildlife refuge, and thus is theoretically a park for wildlife as much as for people.
There is not a great deal of wildlife present, at least in my experience. After all, 160 acres is hardly enough for even a family of deer to live on. The exception to this, of course, are birds, there is supposedly many different types of birds in the park, and indeed, I have seen a blue heron there.
The Willamette itself is probably not the best environment for any form of life, since as, signs note, after it rains, there is an outflow of raw sewage into the river along the course of the park.