I primary spend most of my time on the Mississippi River from pools 4-10 (each pool around 30-40 miles long), however what you will need to understand is that there is cover and there is structure.
Structure is bottom contours, current breaks, eddies and the like.
Cover is an object such as a log or weeds.
You will want to concentrate on the same things that you did in a lake, the difference is that it is going to be much smaller. For example, even a small river will have flats (inside turns) and the difference is instead of being many acres they will be yards instead. Also on the outside bends you will have under cut banks (especially in small rivers) and current breaks around any sharp points or around cover. That cover will also have deeper water on the back side and shallow water on the front side (typically).
Take care,
Steve