John,
Some years back Oregon City decided they didn't want the 
Stergen and Salmon fishermen fishing along the promenade. Some reasons were City liability, trash and public drunkenness. 
So West Linn built the big floats that you mentioned on the West side, just below the locks. Now the fishermen park and hike down to the floats.
The big articulating pipes you see(strong arms) are there to keep docks a safe distance out during low water and also to contend with torrents during floodwaters. The support pipes/tubes  are superior to steel cables because of the huge volume of 
water and flood debris.
B.G.
P.S.   now the promenade is home for the homeless...
you know blue tarps and trash   :-(