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Classes of Rivers

Understanding classification of Rivers

Class A: Lake water. Still. No perceptible movement.
Class I.- Easy. Smooth water; light riffles; clear passages, occasional sand banks and gentle curves. The most difficult problems might arise when paddling around bridges and other obvious obstructions.
Class II.- Moderate. Medium-quick water; rapids with regular waves; clear and open passages between rocks and ledges. Maneuvering required. Best handled by intermediates who can maneuver canoes and read water.
Class III.- Moderately difficult. Numerous high and irregular waves; rocks and eddies with passages clear but narrow and requiring experience to run. Visual inspection required if rapids are unknown. Open canoes without flotation bags will have difficulty. These rapids are best left to canoeists with expert skills.
Class IV- Difficult. Long and powerful rapids and standing waves; souse holes and boiling eddies. Powerful and precise maneuvering required. Visual inspection mandatory. Cannot be run in canoes unless the craft is decked or properly equipped with flotation bags. Advance preparations for possible rescue work important.
Class V- Extremely difficult. Long and violent rapids that follow each other almost without interruption. River filled with obstructions. Big drops and violent currents. Extremely steep gradient. Even reconnoitering may be difficult. Rescue preparations mandatory. Can be run only by top experts in specially equipped whitewater canoes, decked craft, and kayaks.
Class VI.- Extraordinarily difficult. Paddlers face constant threat of death because of extreme danger. Navigable only when water levels and conditions are favorable. This violent whitewater should be left to paddlers of Olympic ability. Every safety precaution must be taken.

This information is from Paddling.net. The full text of the article can be found here.

A good article with some illustrating and exciting video on KayakGuru.com can be found here.