Black Timber, I'm not sure what it's like in Utah but I'm sure it's the same as Here. The rivers are usually open Game. The best way to hunt the rivers are to Find a Boat ramp map. I know a guys doesn't want to give up a hunting saturday but take one morning, Drive the River and Stop by each dock. Check out were the guys are pulling into. Count the rigs. thats a Good indication of were guys like to hunt. From there I would take another day to drive the Waters and Take a map and Detail out were the Decoys are set up at. All those places on the river are Public places unless MArked. Even some of the matked ones might be open, We have a Public MAp that sows us whats private and whats not.
Duckhunting in an area were people hunt alot means you need to Get up 1/2 hour earlier then they do and hit the water running
Without a boat (What it sounds like) You can still access the small islands with a Canoe (If thats on your list). We hunt an Area on the Main river with a Canoe and usually get more birds then the Guys in the MAin Chanel with a billion decoys, the Ducks tend to like the Side pockets and Places they can actually eat and have cover. Any small inlet, away from the current is a good place.
Sorry I don't have any specific details on were to go, But taking one day to drive the River is a good way to find spots.
Another suggestion is find spots you'd like to access and in the Off season, stop by a farmers field of whoever lives there and Just ask. sometimes the Best hunting spots are the Easiest to get too. We found just a spot once... problem being the Guy that lives there is a Talker, And we spend 3 hours trying to get out of there each time
Good luck Mi Amigo........