Actually it's the Multnomah Channel which flows along Sauvie Island.
To get there from Portland you drive US. Route 30 heading towards Sauvie Island Bridge.
Once past the Portland Industrial District (which has a couple cool places to check out and a very interesting history) and a little north you will find people who decided to live on the water. With all the rivers in Oregon it's not surprising there are houseboats in many places. However this area has a very nice view of Sauvie Island.
I would have more photos but it's not easy taking photos of these from land. Would have been a nice day for a sail if I had a boat.
So that is why this Grimm locations post is rather short. Sorry no photos of the specific houseboat from tonight's episode. But then it's safer not to bother the naiads there.