Planning a rafting trip into whitewater rapids? A little homework goes a long way. The international river class (IRC) rating system helps paddlers choose rivers based on their skills and the river’s degree of difficulty. Many organizations borrow from these guidelines.

As class increases, so does the danger and need for good paddling skills, according to American Whitewater, a non-profit that advocates for conservation, restoration and safety.

Generally, river sections with longer and continuous rapids rank higher in class. Small waves and small rocks generally equate to lower class; large waves, boulders and drops deliver higher class. But remember, rivers continually change. That means class III sections could easily become class IV, based on recent weather change, flooding, water level and downed trees.

Class I

Fast-moving water with riffles, small waves and few obstructions, all obvious and easily avoided with little training.

Class II

Straightforward rapids with wide, clear channels, which are evident without scouting.

Class III

Rapids with moderate, irregular waves, which may be difficult to avoid and which can swamp an open canoe.

Class IV

Intense, powerful but predictable rapids requiring precise boat handling in turbulent water.

Class V

Extremely long, obstructed or very violent rapids, which expose a paddler to added risk.

Class VI

Almost never attempted, these runs exemplify the extremes of difficulty, unpredictability and danger in moving water.

Find more detailed information on rapid classes here.