i am waiting to hear back from one of the guys that helped build this kick ass park.
Burnside skatepark was built becuase it was illeagle to skate in downtown portland. So a group of guys decided to build a park. The city wasn't going to build one, so they took it into their own hands to build one. The built it under the burnside bridge without the city's permission. But once the city found out about it, it had already been built, so they let it stay. And there we go.
Burnside is a pretty sketchy place. A lot of weird stuff has gone there, but if you have a pretty big group of friends with you, you should be fine!