There are six degrees of difficulty for rating a rapid or a river, known as "classes," defined below.  It is important to note that the class of a river may change according to the weather, water temperature, water level and geological disturbances, among other things. 
Class I - Moving water with few riffles and small waves. Few or no obstructions. 
Class II - Easy rapids with waves up to three feet and wide clear 
channels that are obvious.