Be real careful of downstream ends of islands as they will tend to have sediment built up from the current. Here is the Columbia River at my Dad's house: http://binged.it/14eKZA3
The current runs top to bottom. Look at the upstream part of the bottom island versus the downstream part of the upper island. When navigating through this part you want to be closer to the upper island. Although note that the upper island has a small sandbar on the downstream part.
If you notice the depth getting really shallow don't get off the gas as that will lower the boat's depth in the water but try to turn towards deeper water. Typically the river bottom will change gradually so watch trends on your depth finder.
Make sure you have a anchor in your boat in case your have mechanical problems.
Be careful with other boats because while everyone only drives in 2 directions you are typically closer to each other then on a lake. If your not sure what to do you can always just stop. Keep your rider safe.
Having a local tell you where it is safe and where it isn't can save you a prop repair or worse. A family friend showed me this part of the river and I have a 10 mile stretch where I know where to go and I don't go beyond that.
If your river has commercial traffic they have right of way if their draft/equipment restricts where they can go.
In general I like river riding better then lakes because the current will smooth out the water after a boat/rider comes through in a minute or so.