A skier is aware that a black diamond run is a lot steeper and more difficult than a green circle slope. Rapids, like ski slopes, vary in their intensity. The International Rating system classifies rapids as follows:

Class A -  Lake water. Still. No perceptible movement. met.

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. classification.

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.

United States Grading System

The specific grading system in the United States of America is an expanded, more detailed version of the international scale, which is adopted or preferred by many other national whitewater organizations.

Grade I: Fast moving water with riffles and small waves. Few obstructions, all obvious and easily missed with little training. Risk to swimmers is slight; self-rescue is easy.

Grade II: Straightforward rapids with wide, clear channels which are evident without scouting. Occasional maneuvering may be required, but rocks and medium-sized waves are easily missed by trained paddlers. Swimmers are seldom injured and group assistance, while helpful, is seldom needed. Rapids that are at the upper end of this difficulty range are designated “Class II+”.

Grade III: Rapids with moderate, irregular waves which may be difficult to avoid and which can swamp an open canoe. Complex maneuvers in fast current and good boat control in tight passages or around ledges are often required; large waves or strainers may be present but are easily avoided. Strong eddies and powerful current effects can be found, particularly on large-volume rivers. Scouting is advisable for inexperienced parties. Injuries while swimming are rare; self-rescue is usually easy but group assistance may be required to avoid long swims. Rapids that are at the lower or upper end of this difficulty range are designated “Class III-” or “Class III+” respectively.

Grade IV: Intense, powerful but predictable rapids requiring precise boat handling in turbulent water. Depending on the character of the river, it may feature large, unavoidable waves and holes or constricted passages demanding fast maneuvers under pressure. A fast, reliable eddy turn may be needed to initiate maneuvers, scout rapids, or rest. Rapids may require “must” moves above dangerous hazards. Scouting may be necessary the first time down. Risk of injury to swimmers is moderate to high, and water conditions may make self-rescue difficult. Group assistance for rescue is often essential but requires practiced skills. A strong kayak roll is highly recommended. Rapids that are at the lower or upper end of this difficulty range are designated “Class IV-” or “Class IV+” respectively. Limit of open top canoes.

Grade V: Extremely long, obstructed, or very violent rapids which expose a paddler to added risk. Drops may contain large, unavoidable waves and holes or steep, congested chutes with complex, demanding routes. Rapids may continue for long distances between pools, demanding a high level of fitness. What eddies exist may be small, turbulent, or difficult to reach. At the high end of the scale, several of these factors may be combined. Scouting is recommended but may be difficult. Swims are dangerous, and rescue is often difficult even for experts. A very reliable kayak roll, proper equipment, extensive experience, and practiced rescue skills are essential. Because of the large range of difficulty that exists beyond Class IV, Class 5 is an open-ended, multiple-level scale designated by class 5.0, 5.1, 5.2, etc... each of these levels is an order of magnitude more difficult than the last. Example: increasing difficulty from Class 5.0 to Class 5.1 is a similar order of magnitude as increasing from Class IV to Class 5.0.

Grade VI: These runs have almost never been attempted and often exemplify the extremes of difficulty, unpredictability and danger. The consequences of errors are very severe and rescue may be impossible. For teams of experts only, at favorable water levels, after close personal inspection and taking all precautions. After a Class VI rapids has been run many times, its rating may be changed to an appropriate Class 5.x rating.

The characteristics of a river can change remarkably as the water level rises or falls. As you might expect, a set of Class II rapids can become raging Class IV when the water is abnormally high following spring runoff or heavy storms. O the other hand, a Class IV can turn into a shallow puddle when the water level is low in the late summer. Even normally calm stretches become turbulent and dangerous at flood stage, because the force of currents slammed this way and that by rocks and obstructions creates powerful and dangerous surface conditions.

An International Rating system has also been devised to describe river flow. The classification for a specific river may change from season to season; the following letter designations are used to describe water level and rate of flow:

L or Low. Below-normal levels for the river. Below-normal depth may interfere with good paddling. Shallows may turn into dry banks and low areas become muddy sandbars.

M or Medium. Normal river flow. Medium water generally is used to describe good water for rivers with slight gradients and enough depth for passage on the steeper sections.

MH or Medium High. Higher than normal. Faster flow on gentle gradients. The best flow for more difficult river sections with enough water for passage over low ledges and through rock gardens.

H or High. Water is becoming difficult to handle. he river is well above normal stage. Canoeists may refer to the strong currents as "heavy." Small debris may come floating by, a warning that the river is dangerous and better left to skilled kayakers or canoeists whose craft are supported by flotation bags.

HH or High-High. Very heavy water. Hydraulics are complex. Even slight gradients become treacherous. Debris frequent. Only for experts.