Good Morning,
I have spent a lot of time on Sauvie Island for vacations. Just wanted to share my impressions:
There is one car bridge on and off the island. As soon as you come over the
bridge, you can turn either left or right, and the road circles around the island and meets up back at the same spot. There is a little store, called "The Cracker Barrell" just to the left as you come off the bridge. To use the recreation facilities on the island, ie the beach, you have to buy either a day pass or season pass from either that store, or the only other little store on the island which is a few miles away, toward the public beaches. You go inside, pay your money, and you get a slip to put in your window. I guess it is technically a parking permit, and they will tow you if it's not in your window. It is to park at the beach areas.
Keep following the road to the right, and you will see expensive looking houseboats on your left. A couple of farms with sales outlets are on the right, the scenary on the island is bucolic and has been referred to in the New York Times as Oregon's "Martha's Vineyard". You pass lots of farms and houses, horticultural farms and acres and acres of corn.
We turn off the right and go left at the next stop. We follow this road along to the north and east of SI. The public beaches are just where the paved road turns into a gravel road. You have to park along the gravel, then cross the road to get to the beach areas. There are a lot of woods on the beach side, but the road side is mostly fields. There are some dog trial hunting training type places further west.
There are several marked entrances to the public beaches. You pretty much have to follow the paths to the beaches the further north you park along the road as the trees get thicker.
For what it's worth (and I like to go there sometimes) there is a public clothing-optional beach, called Collins Beach, further north on the beach road. Lots of people go there, including families with children, old folks, and just regular middle-age ladies like me who have fibromyalgia and need a break from clothes once and a while. There is an organization that sort of watches over what happens there (ORCOBA) and it is most definitely not about sex or sexuality for most people. In fact, anyone who starts staring too long usually gets a reproach from another beach person.
FWIW, as someone who does not live on the island, it is definitely a destination type place, not a just "passing thru" place.
Go, sleuths, go!