I don't know how they line up with the dams but the Lewis and Clark journals as well as some latter trappers mention a lot about rapids and portages in the area north and west of the Blue Mountains.
Bonneville also tried to get to the Willamette valley in winter and hoped to go down the snake on foot, but found it impassable and had to trudge over the blues, through the Grand Rhonde valley and back down the snake somehow. Finally got back to the river and couldn't make a go of it - had to back track to their winter camp (somewhere on the upper salmon near north fork, id - if I recall correctly.
I think there were some major rapids and falls mentioned in his journals around the 1000 springs area, Fascinating reading if you're into early Anglo-European history of the region. I just don't have much of a handle on where the dams are and what remains. It's not an area I've been in much, except on the reservoirs when I was a kid.
One issue with removal will be the sedimentation/down cutting. Think Pearce Ferry rapid videos of late. When I was young that was a life's goal of mine, to work with the morphological changes of streams as the system re-equilibrated following dam removal. Needless to say, it hasn't become much of a career path yet... maybe there is still hope. Anyways it may take some time for things to "settle down" following any major dam removal prior to it being safe to start boating in earnest.
Yesterday's gone on down the river and you can't get it back. - Agustus McCrae