- Class 1: Very small rough areas, requires no maneuvering. (Skill Level: None)
- Class 2: Some rough water, maybe some rocks, small drops, might require maneuvering. (Skill Level: Basic Paddling Skill)
- Class 3: Whitewater, medium waves, maybe a 3–5 ft drop, but not much considerable danger. May require significant maneuvering. (Skill Level: Experienced paddling skills)
- Class 4: Whitewater, large waves, long rapids, rocks, maybe a considerable drop, sharp maneuvers may be needed. (Skill Level: Whitewater Experience)
- Class 5: Whitewater, large waves, continuous rapids, large rocks and hazards, maybe a large drop, precise maneuvering (Skill Level: Advanced Whitewater Experience)
- Class 6: Whitewater, typically with huge waves, huge rocks and hazards, huge drops, but sometimes labeled this way due to largely invisible dangers (e.g., a smooth slide that creates a near-perfect, almost inescapable hydraulic, as at Woodall Shoals or Chattooga). Class 6 rapids are considered hazardous even for expert paddlers using state-of-the-art equipment, and come with the warning "danger to life or limb." (Skill Level: Expert)
The danger of capsizing your raft generally starts with a class 3 and gets progressively greater.
Its actually a quite extensive article and worth a read for anyone wanting more information on terminology commonly used in rafting.
Obviously after heavy rains, the flow of the river will be greater and the classification will generally increase. Spring and early summer tend to be a time where mountain/valley rivers are at their strongest due to melting snow and ice from higher elevations.
Before I ever go Kayaking, I always call a local guide and ask how the flow is.