This is a basic guide to river classification. Every river is different, and different factors can influence classifiation (this includes length of rapid, weather conditions, wilderness considerations and type of boat).
We often use a + or - to indicate that a rapid is between classes.

Class I
Clear, wide channels with small waves. No obstructions or hazards.

Class II
Slightly larger, more frequent waves. Little or no obstructions or hazards. Little Maneuvering required.

Class III
Waves up to four feet in height. Some obstructions present. Maneuvering required.

Class IV
Difficult with large frequent waves and obstructions. Skillful maneuvering required.

Class V
Large frequent waves which are often unavoidable. Techincal rapids with many hazards and obstructions present.

Class VI
The upper limit of navigation attempted only career risk takers.