I've caught some of my biggest largemouths in rivers. Try to use anything that is native to that river and will resemble what they normally eat, such as craws, creatures, baitfish, etc. If fishing for smallmouth, cast behind a rock in the current where they would be hanging out in a pool. If fishing for largemouth, always try and find structure. Side streams and flowing water from other sources usually provide big fish as well. Casting into the side stream and having it float down into the main river almost always yields strikes for me. Experiment - see what works best for you and what you like best. If you have any other specific questions, feel free to PM me!