CURRENT. CURRENT. CURRENT.
I grew up fishing fast moving mountain rivers for smallmouth, then moved South, next to a deep, slow moving river full of largemouth. Current is the biggest factor in both circumstances.
For river largemouth, we have a progression that we look for:
1. Mixing water. If we can find clear water pouring out of a creek, or even a trickle of "black" water oozing into our stained river, we've probably found the active bass. The bait fish hide in the breaklines, and the active bass flock to them like a fat man to the dinner Bell. Our best presentations here are crankbaits, spinnerbaits, and swimbaits. The lure typically isn't as important as matching the color to the water (use colors appropriate for the dingier main river).
2. Outside bends. Over time the water will wash our little "caves" in these banks. Bass love to back up into them, and rest in the shade and current break. They will take an easy meal if it presents itself. Our best presentations are jigs and Texas rigged plastics.
3. Cover. Fallen trees, cypress knees, bridge pilings, boat docks, rocks, grass clumps, etc. Largemouth are lazy, and tend to use the cover as a current break to get a little rest. There are two great presentations here. If the fish are active (mornings, evenings, prespawn, Fall frenzy, etc.), try bringing a moving bait to the fish from an upstream position. If the fish are inactive (midday, postspawn, summer when it's just hot as blue blazes), try a virtical presentation. I like to drop a jig or plastic right on their nose.
Now, when you find a spot that has two of those factors, you've found gold. If you find all three, you've found fish heaven!
Of course, none of this means a thing if you don't pay attention to the current.
One of our best holes is a creek (that often creates mixing water), on an outside bend, that has a few fallen trees below the surface. All of these factors combine to form the perfect storm of current breaks. We try to cast just beyond any little current breaks, eddies, or slackwater spots, and bring out baits through them.
River largemouth are a blast! They're typically a little shorter than our lake bass in the, but they're stockier. I presume this is from the constant workout the current provides. Whatever the reason, they fight more like smallmouth, and love to use the current to get away from the ugly guyin my avatar!
Good luck!
