Someone may know better, but it appears to be free:
Parks & Recreation
Located under the St. Johns Bridge, Cathedral Park gives residents an opportunity to explore over 23 acres of natural wildlife habitat. A summer Jazz Festival, off-leash area for dogs, boat ramp and Wi-Fi access are all free to anyone visiting this historic day park. The St. Johns Community Garden is also a great place to enjoy the best in urban greenspaces.
That's funny, I saw the post before you put the "u" in and it looked like something totally different. I was like "um...ok?"
You might want to confirm with another Portlander that has used it this year. I launched with a friend a couple years ago and it was free, and it looks like it still is, but you may want to get a 2nd opinion just to make sure.