Fish the Calm Waters
The bays and backwaters will usually be home to the better fishing for sunfish on the rivers. Find some vegetation back there and you may find a nice spot to catch some sunnies.
Find the Current Breaks
The current breaks are key when river fishing. Sunfish can be found in current, but they prefer to have little to no current. The current breaks are great because all kinds of food gets washed on by and the sunfish can easily go into the current to grab a meal then back into the calmer waters to rest.
Bridges are Great Spots to Fish
Bridges are good fish attractors and sunfish will use them too. You can usually find the sunfish stacked up behind the bridge pilings in the slack water.