or levels, generally running from I to VI on basis of how navigable the rapids are. A Class 5 rapid may be categorized as Class 5.1-5.9. While Class I rapids are easy to navigate and require little maneuvering, Class VI rapids pose threat to life with little or no chance for rescue, often classified as 'U' for Unraftable. River rafting sports are carried out where many rapids are present in the course.

Constriction refers to when rivers flow through narrower channels, thus increasing the velocity