cover, concealment, forage, and change in current with change in depth.

minnows follow the bait

small mouth are current related large mouth are not so they like slack water or back water areas. trees. Lilly pads rushes any thing that offers them a place to ambush from .
if you fish for them in a lake the same things will attract them in rivers .

here where I live the river bass like to hide behind big bolders in the river in rapids they cause a slack spot in current, I toss a minnow type lure behind the boulders. Nine times out of ten theres's big small mouth hiding there also look for areas where there is a gravel bottom small mouth will get in these areas looking for crawdads,helgramites,worms etc. mcopeland1

The number one thing is rocks. Don't fish mud bottom for bass. Unless the entire river is mud bottom, but I dont think I've ever seen that before. For largemouths backwaters and creeks are probably best while smallmouths will be in the main river more. If you are pitching tight to cover try something crawdad imitating because they are a primary forage of river fish. If throwing hardbaits and trying to cover water make sure you have rattles. Bass have to be able to find the bait in loud moving water and all the other noises associated with rivers.

If you are fishing a river try to fish the areas where smaller creeks and rivers empty into the main body of water, there are always a couple of bass laying in those cuts ambushing bait fish. If the bass are small in that area and you are tournament fishing then try another location because the larger bass usually run off smaller fish.

I disagree about fishing mud flats and bottoms, at least in my area. It is quit common here to find bass on mud bottom but usually only near a hole, drop off or channel. I caught my largest bass (5.5 pounds) on a mud bottom near the main channel in Stones River southeast of Nashville.